.\" Copyright (C) 2014 Kees Cook <keescook@chromium.org>

.\" and Copyright (C) 2012 Will Drewry <wad@chromium.org>

.\" and Copyright (C) 2008, 2014,2017 Michael Kerrisk <mtk.manpages@gmail.com>

.\" and Copyright (C) 2017 Tyler Hicks <tyhicks@canonical.com>

.\"

.\" %%%LICENSE_START(VERBATIM)

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.\" manual provided the copyright notice and this permission notice are

.\" preserved on all copies.

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.\" Permission is granted to copy and distribute modified versions of this

.\" manual under the conditions for verbatim copying, provided that the

.\" entire resulting derived work is distributed under the terms of a

.\" permission notice identical to this one.

.\"

.\" Since the Linux kernel and libraries are constantly changing, this

.\" manual page may be incorrect or out-of-date.  The author(s) assume no

.\" responsibility for errors or omissions, or for damages resulting from

.\" the use of the information contained herein.  The author(s) may not

.\" have taken the same level of care in the production of this manual,

.\" which is licensed free of charge, as they might when working

.\" professionally.

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.\" %%%LICENSE_END

.\"

.TH SECCOMP 2 2018-02-02 "Linux" "Linux Programmer's Manual"

.SH NAME

seccomp \- operate on Secure Computing state of the process

.SH SYNOPSIS

.nf

.B #include <linux/seccomp.h>

.B #include <linux/filter.h>

.B #include <linux/audit.h>

.B #include <linux/signal.h>

.B #include <sys/ptrace.h>

.\" Kees Cook noted: Anything that uses SECCOMP_RET_TRACE returns will

.\"                  need <sys/ptrace.h>

.PP

.BI "int seccomp(unsigned int " operation ", unsigned int " flags \

", void *" args );

.fi

.SH DESCRIPTION

The

.BR seccomp ()

system call operates on the Secure Computing (seccomp) state of the

calling process.

.PP

Currently, Linux supports the following

.IR operation

values:

.TP

.BR SECCOMP_SET_MODE_STRICT

The only system calls that the calling thread is permitted to make are

.BR read (2),

.BR write (2),

.BR _exit (2)

(but not

.BR exit_group (2)),

and

.BR sigreturn (2).

Other system calls result in the delivery of a

.BR SIGKILL

signal.

Strict secure computing mode is useful for number-crunching

applications that may need to execute untrusted byte code, perhaps

obtained by reading from a pipe or socket.

.IP

Note that although the calling thread can no longer call

.BR sigprocmask (2),

it can use

.BR sigreturn (2)

to block all signals apart from

.BR SIGKILL

and

.BR SIGSTOP .

This means that

.BR alarm (2)

(for example) is not sufficient for restricting the process's execution time.

Instead, to reliably terminate the process,

.BR SIGKILL

must be used.

This can be done by using

.BR timer_create (2)

with

.BR SIGEV_SIGNAL

and

.IR sigev_signo

set to

.BR SIGKILL ,

or by using

.BR setrlimit (2)

to set the hard limit for

.BR RLIMIT_CPU .

.IP

This operation is available only if the kernel is configured with

.BR CONFIG_SECCOMP

enabled.

.IP

The value of

.IR flags

must be 0, and

.IR args

must be NULL.

.IP

This operation is functionally identical to the call:

.IP

    prctl(PR_SET_SECCOMP, SECCOMP_MODE_STRICT);

.TP

.BR SECCOMP_SET_MODE_FILTER

The system calls allowed are defined by a pointer to a Berkeley Packet

Filter (BPF) passed via

.IR args .

This argument is a pointer to a

.IR "struct\ sock_fprog" ;

it can be designed to filter arbitrary system calls and system call

arguments.

If the filter is invalid,

.BR seccomp ()

fails, returning

.BR EINVAL

in

.IR errno .

.IP

If

.BR fork (2)

or

.BR clone (2)

is allowed by the filter, any child processes will be constrained to

the same system call filters as the parent.

If

.BR execve (2)

is allowed,

the existing filters will be preserved across a call to

.BR execve (2).

.IP

In order to use the

.BR SECCOMP_SET_MODE_FILTER

operation, either the caller must have the

.BR CAP_SYS_ADMIN

capability in its user namespace, or the thread must already have the

.I no_new_privs

bit set.

If that bit was not already set by an ancestor of this thread,

the thread must make the following call:

.IP

    prctl(PR_SET_NO_NEW_PRIVS, 1);

.IP

Otherwise, the

.BR SECCOMP_SET_MODE_FILTER

operation fails and returns

.BR EACCES

in

.IR errno .

This requirement ensures that an unprivileged process cannot apply

a malicious filter and then invoke a set-user-ID or

other privileged program using

.BR execve (2),

thus potentially compromising that program.

(Such a malicious filter might, for example, cause an attempt to use

.BR setuid (2)

to set the caller's user IDs to nonzero values to instead

return 0 without actually making the system call.

Thus, the program might be tricked into retaining superuser privileges

in circumstances where it is possible to influence it to do

dangerous things because it did not actually drop privileges.)

.IP

If

.BR prctl (2)

or

.BR seccomp ()

is allowed by the attached filter, further filters may be added.

This will increase evaluation time, but allows for further reduction of

the attack surface during execution of a thread.

.IP

The

.BR SECCOMP_SET_MODE_FILTER

operation is available only if the kernel is configured with

.BR CONFIG_SECCOMP_FILTER

enabled.

.IP

When

.IR flags

is 0, this operation is functionally identical to the call:

.IP

    prctl(PR_SET_SECCOMP, SECCOMP_MODE_FILTER, args);

.IP

The recognized

.IR flags

are:

.RS

.TP

.BR SECCOMP_FILTER_FLAG_TSYNC

When adding a new filter, synchronize all other threads of the calling

process to the same seccomp filter tree.

A "filter tree" is the ordered list of filters attached to a thread.

(Attaching identical filters in separate

.BR seccomp ()

calls results in different filters from this perspective.)

.IP

If any thread cannot synchronize to the same filter tree,

the call will not attach the new seccomp filter,

and will fail, returning the first thread ID found that cannot synchronize.

Synchronization will fail if another thread in the same process is in

.BR SECCOMP_MODE_STRICT

or if it has attached new seccomp filters to itself,

diverging from the calling thread's filter tree.

.TP

.BR SECCOMP_FILTER_FLAG_LOG " (since Linux 4.14)"

.\" commit e66a39977985b1e69e17c4042cb290768eca9b02

All filter return actions except

.BR SECCOMP_RET_ALLOW

should be logged.

An administrator may override this filter flag by preventing specific

actions from being logged via the

.IR /proc/sys/kernel/seccomp/actions_logged

file.

.RE

.TP

.BR SECCOMP_GET_ACTION_AVAIL " (since Linux 4.14)"

.\" commit d612b1fd8010d0d67b5287fe146b8b55bcbb8655

Test to see if an action is supported by the kernel.

This operation is helpful to confirm that the kernel knows

of a more recently added filter return action

since the kernel treats all unknown actions as

.BR SECCOMP_RET_KILL_PROCESS .

.IP

The value of

.IR flags

must be 0, and

.IR args

must be a pointer to an unsigned 32-bit filter return action.

.SS Filters

When adding filters via

.BR SECCOMP_SET_MODE_FILTER ,

.IR args

points to a filter program:

.PP

.in +4n

.EX

struct sock_fprog {

    unsigned short      len;    /* Number of BPF instructions */

    struct sock_filter *filter; /* Pointer to array of

                                   BPF instructions */

};

.EE

.in

.PP

Each program must contain one or more BPF instructions:

.PP

.in +4n

.EX

struct sock_filter {            /* Filter block */

    __u16 code;                 /* Actual filter code */

    __u8  jt;                   /* Jump true */

    __u8  jf;                   /* Jump false */

    __u32 k;                    /* Generic multiuse field */

};

.EE

.in

.PP

When executing the instructions, the BPF program operates on the

system call information made available (i.e., use the

.BR BPF_ABS

addressing mode) as a (read-only)

.\" Quoting Kees Cook:

.\"     If BPF even allows changing the data, it's not copied back to

.\"     the syscall when it runs. Anything wanting to do things like

.\"     that would need to use ptrace to catch the call and directly

.\"     modify the registers before continuing with the call.

buffer of the following form:

.PP

.in +4n

.EX

struct seccomp_data {

    int   nr;                   /* System call number */

    __u32 arch;                 /* AUDIT_ARCH_* value

                                   (see <linux/audit.h>) */

    __u64 instruction_pointer;  /* CPU instruction pointer */

    __u64 args[6];              /* Up to 6 system call arguments */

};

.EE

.in

.PP

Because numbering of system calls varies between architectures and

some architectures (e.g., x86-64) allow user-space code to use

the calling conventions of multiple architectures, it is usually

necessary to verify the value of the

.IR arch

field.

.PP

It is strongly recommended to use a whitelisting approach whenever

possible because such an approach is more robust and simple.

A blacklist will have to be updated whenever a potentially

dangerous system call is added (or a dangerous flag or option if those

are blacklisted), and it is often possible to alter the

representation of a value without altering its meaning, leading to

a blacklist bypass.

See also

.IR Caveats

below.

.PP

The

.IR arch

field is not unique for all calling conventions.

The x86-64 ABI and the x32 ABI both use

.BR AUDIT_ARCH_X86_64

as

.IR arch ,

and they run on the same processors.

Instead, the mask

.BR __X32_SYSCALL_BIT

is used on the system call number to tell the two ABIs apart.

.\" As noted by Dave Drysdale in a note at the end of

.\" https://lwn.net/Articles/604515/

.\"     One additional detail to point out for the x32 ABI case:

.\"     the syscall number gets a high bit set (__X32_SYSCALL_BIT),

.\"     to mark it as an x32 call.

.\"

.\"     If x32 support is included in the kernel, then __SYSCALL_MASK

.\"     will have a value that is not all-ones, and this will trigger

.\"     an extra instruction in system_call to mask off the extra bit,

.\"     so that the syscall table indexing still works.

.PP

This means that in order to create a seccomp-based

blacklist for system calls performed through the x86-64 ABI,

it is necessary to not only check that

.IR arch

equals

.BR AUDIT_ARCH_X86_64 ,

but also to explicitly reject all system calls that contain

.BR __X32_SYSCALL_BIT

in

.IR nr .

.PP

The

.I instruction_pointer

field provides the address of the machine-language instruction that

performed the system call.

This might be useful in conjunction with the use of

.I /proc/[pid]/maps

to perform checks based on which region (mapping) of the program

made the system call.

(Probably, it is wise to lock down the

.BR mmap (2)

and

.BR mprotect (2)

system calls to prevent the program from subverting such checks.)

.PP

When checking values from

.IR args

against a blacklist, keep in mind that arguments are often

silently truncated before being processed, but after the seccomp check.

For example, this happens if the i386 ABI is used on an

x86-64 kernel: although the kernel will normally not look beyond

the 32 lowest bits of the arguments, the values of the full

64-bit registers will be present in the seccomp data.

A less surprising example is that if the x86-64 ABI is used to perform

a system call that takes an argument of type

.IR int ,

the more-significant half of the argument register is ignored by

the system call, but visible in the seccomp data.

.PP

A seccomp filter returns a 32-bit value consisting of two parts:

the most significant 16 bits

(corresponding to the mask defined by the constant

.BR SECCOMP_RET_ACTION_FULL )

contain one of the "action" values listed below;

the least significant 16-bits (defined by the constant

.BR SECCOMP_RET_DATA )

are "data" to be associated with this return value.

.PP

If multiple filters exist, they are \fIall\fP executed,

in reverse order of their addition to the filter tree\(emthat is,

the most recently installed filter is executed first.

(Note that all filters will be called

even if one of the earlier filters returns

.BR SECCOMP_RET_KILL .

This is done to simplify the kernel code and to provide a

tiny speed-up in the execution of sets of filters by

avoiding a check for this uncommon case.)

.\" From an Aug 2015 conversation with Kees Cook where I asked why *all*

.\" filters are applied even if one of the early filters returns

.\" SECCOMP_RET_KILL:

.\"

.\"     It's just because it would be an optimization that would only speed up

.\"     the RET_KILL case, but it's the uncommon one and the one that doesn't

.\"     benefit meaningfully from such a change (you need to kill the process

.\"     really quickly?). We would speed up killing a program at the (albeit

.\"     tiny) expense to all other filtered programs. Best to keep the filter

.\"     execution logic clear, simple, and as fast as possible for all

.\"     filters.

The return value for the evaluation of a given system call is the first-seen

action value of highest precedence (along with its accompanying data)

returned by execution of all of the filters.

.PP

In decreasing order of precedence,

the action values that may be returned by a seccomp filter are:

.TP

.BR SECCOMP_RET_KILL_PROCESS " (since Linux 4.14)"

.\" commit 4d3b0b05aae9ee9ce0970dc4cc0fb3fad5e85945

.\" commit 0466bdb99e8744bc9befa8d62a317f0fd7fd7421

This value results in immediate termination of the process,

with a core dump.

The system call is not executed.

By contrast with

.BR SECCOMP_RET_KILL_THREAD

below, all threads in the thread group are terminated.

(For a discussion of thread groups, see the description of the

.BR CLONE_THREAD

flag in

.BR clone (2).)

.IP

The process terminates

.I "as though"

killed by a

.B SIGSYS

signal.

Even if a signal handler has been registered for

.BR SIGSYS ,

the handler will be ignored in this case and the process always terminates.

To a parent process that is waiting on this process (using

.BR waitpid (2)

or similar), the returned

.I wstatus

will indicate that its child was terminated as though by a

.BR SIGSYS

signal.

.TP

.BR SECCOMP_RET_KILL_THREAD " (or " SECCOMP_RET_KILL )

This value results in immediate termination of the thread

that made the system call.

The system call is not executed.

Other threads in the same thread group will continue to execute.

.IP

The thread terminates

.I "as though"

killed by a

.B SIGSYS

signal.

See

.BR SECCOMP_RET_KILL_PROCESS

above.

.IP

.\" See these commits:

.\" seccomp: dump core when using SECCOMP_RET_KILL

.\"    (b25e67161c295c98acda92123b2dd1e7d8642901)

.\" seccomp: Only dump core when single-threaded

.\"    (d7276e321ff8a53106a59c85ca46d03e34288893)

Before Linux 4.11,

any process terminated in this way would not trigger a coredump

(even though

.B SIGSYS

is documented in

.BR signal (7)

as having a default action of termination with a core dump).

Since Linux 4.11,

a single-threaded process will dump core if terminated in this way.

.IP

With the addition of

.BR SECCOMP_RET_KILL_PROCESS

in Linux 4.14,

.BR SECCOMP_RET_KILL_THREAD

was added as a synonym for

.BR SECCOMP_RET_KILL ,

in order to more clearly distinguish the two actions.

.TP

.BR SECCOMP_RET_TRAP

This value results in the kernel sending a thread-directed

.BR SIGSYS

signal to the triggering thread.

(The system call is not executed.)

Various fields will be set in the

.I siginfo_t

structure (see

.BR sigaction (2))

associated with signal:

.RS

.IP * 3

.I si_signo

will contain

.BR SIGSYS .

.IP *

.IR si_call_addr

will show the address of the system call instruction.

.IP *

.IR si_syscall

and

.IR si_arch

will indicate which system call was attempted.

.IP *

.I si_code

will contain

.BR SYS_SECCOMP .

.IP *

.I si_errno

will contain the

.BR SECCOMP_RET_DATA

portion of the filter return value.

.RE

.IP

The program counter will be as though the system call happened

(i.e., the program counter will not point to the system call instruction).

The return value register will contain an architecture\-dependent value;

if resuming execution, set it to something appropriate for the system call.

(The architecture dependency is because replacing it with

.BR ENOSYS

could overwrite some useful information.)

.TP

.BR SECCOMP_RET_ERRNO

This value results in the

.B SECCOMP_RET_DATA

portion of the filter's return value being passed to user space as the

.IR errno

value without executing the system call.

.TP

.BR SECCOMP_RET_TRACE

When returned, this value will cause the kernel to attempt to notify a

.BR ptrace (2)-based

tracer prior to executing the system call.

If there is no tracer present,

the system call is not executed and returns a failure status with

.I errno

set to

.BR ENOSYS .

.IP

A tracer will be notified if it requests

.BR PTRACE_O_TRACESECCOMP

using

.IR ptrace(PTRACE_SETOPTIONS) .

The tracer will be notified of a

.BR PTRACE_EVENT_SECCOMP

and the

.BR SECCOMP_RET_DATA

portion of the filter's return value will be available to the tracer via

.BR PTRACE_GETEVENTMSG .

.IP

The tracer can skip the system call by changing the system call number

to \-1.

Alternatively, the tracer can change the system call

requested by changing the system call to a valid system call number.

If the tracer asks to skip the system call, then the system call will

appear to return the value that the tracer puts in the return value register.

.IP

.\" This was changed in ce6526e8afa4.

.\" A related hole, using PTRACE_SYSCALL instead of SECCOMP_RET_TRACE, was

.\" changed in arch-specific commits, e.g. 93e35efb8de4 for X86 and

.\" 0f3912fd934c for ARM.

Before kernel 4.8, the seccomp check will not be run again after the tracer is

notified.

(This means that, on older kernels, seccomp-based sandboxes

.B "must not"

allow use of

.BR ptrace (2)\(emeven

of other

sandboxed processes\(emwithout extreme care;

ptracers can use this mechanism to escape from the seccomp sandbox.)

.TP

.BR SECCOMP_RET_LOG " (since Linux 4.14)"

.\" commit 59f5cf44a38284eb9e76270c786fb6cc62ef8ac4

This value results in the system call being executed after

the filter return action is logged.

An administrator may override the logging of this action via

the

.IR /proc/sys/kernel/seccomp/actions_logged

file.

.TP

.BR SECCOMP_RET_ALLOW

This value results in the system call being executed.

.PP

If an action value other than one of the above is specified,

then the filter action is treated as either

.BR SECCOMP_RET_KILL_PROCESS

(since Linux 4.14)

.\" commit 4d3b0b05aae9ee9ce0970dc4cc0fb3fad5e85945

or

.BR SECCOMP_RET_KILL_THREAD

(in Linux 4.13 and earlier).

.\"

.SS /proc interfaces

The files in the directory

.IR /proc/sys/kernel/seccomp

provide additional seccomp information and configuration:

.TP

.IR actions_avail " (since Linux 4.14)"

.\" commit 8e5f1ad116df6b0de65eac458d5e7c318d1c05af

A read-only ordered list of seccomp filter return actions in string form.

The ordering, from left-to-right, is in decreasing order of precedence.

The list represents the set of seccomp filter return actions

supported by the kernel.

.TP

.IR actions_logged " (since Linux 4.14)"

.\" commit 0ddec0fc8900201c0897b87b762b7c420436662f

A read-write ordered list of seccomp filter return actions that

are allowed to be logged.

Writes to the file do not need to be in ordered form but reads from

the file will be ordered in the same way as the

.IR actions_avail

file.

.IP

It is important to note that the value of

.IR actions_logged

does not prevent certain filter return actions from being logged when

the audit subsystem is configured to audit a task.

If the action is not found in the

.IR actions_logged

file, the final decision on whether to audit the action for that task is

ultimately left up to the audit subsystem to decide for all filter return

actions other than

.BR SECCOMP_RET_ALLOW .

.IP

The "allow" string is not accepted in the

.IR actions_logged

file as it is not possible to log

.BR SECCOMP_RET_ALLOW

actions.

Attempting to write "allow" to the file will fail with the error

.BR EINVAL .

.\"

.SS Audit logging of seccomp actions

.\" commit 59f5cf44a38284eb9e76270c786fb6cc62ef8ac4

Since Linux 4.14, the kernel provides the facility to log the

actions returned by seccomp filters in the audit log.

The kernel makes the decision to log an action based on

the action type,  whether or not the action is present in the

.I actions_logged

file, and whether kernel auditing is enabled

(e.g., via the kernel boot option

.IR audit=1 ).

.\" or auditing could be enabled via the netlink API (AUDIT_SET)

The rules are as follows:

.IP * 3

If the action is

.BR SECCOMP_RET_ALLOW ,

the action is not logged.

.IP *

Otherwise, if the action is either

.BR SECCOMP_RET_KILL_PROCESS

or

.BR SECCOMP_RET_KILL_THREAD ,

and that action appears in the

.IR actions_logged

file, the action is logged.

.IP *

Otherwise, if the filter has requested logging (the

.BR SECCOMP_FILTER_FLAG_LOG

flag)

and the action appears in the

.IR actions_logged

file, the action is logged.

.IP *

Otherwise, if kernel auditing is enabled and the process is being audited

.RB ( autrace (8)),

the action is logged.

.IP *

Otherwise, the action is not logged.

.SH RETURN VALUE

On success,

.BR seccomp ()

returns 0.

On error, if

.BR SECCOMP_FILTER_FLAG_TSYNC

was used,

the return value is the ID of the thread

that caused the synchronization failure.

(This ID is a kernel thread ID of the type returned by

.BR clone (2)

and

.BR gettid (2).)

On other errors, \-1 is returned, and

.IR errno

is set to indicate the cause of the error.

.SH ERRORS

.BR seccomp ()

can fail for the following reasons:

.TP

.BR EACCESS

The caller did not have the

.BR CAP_SYS_ADMIN

capability in its user namespace, or had not set

.IR no_new_privs

before using

.BR SECCOMP_SET_MODE_FILTER .

.TP

.BR EFAULT

.IR args

was not a valid address.

.TP

.BR EINVAL

.IR operation

is unknown or is not supported by this kernel version or configuration.

.TP

.B EINVAL

The specified

.IR flags

are invalid for the given

.IR operation .

.TP

.BR EINVAL

.I operation

included

.BR BPF_ABS ,

but the specified offset was not aligned to a 32-bit boundary or exceeded

.IR "sizeof(struct\ seccomp_data)" .

.TP

.BR EINVAL

.\" See kernel/seccomp.c::seccomp_may_assign_mode() in 3.18 sources

A secure computing mode has already been set, and

.I operation

differs from the existing setting.

.TP

.BR EINVAL

.I operation

specified

.BR SECCOMP_SET_MODE_FILTER ,

but the filter program pointed to by

.I args

was not valid or the length of the filter program was zero or exceeded

.B BPF_MAXINSNS

(4096) instructions.

.TP

.BR ENOMEM

Out of memory.

.TP

.BR ENOMEM

.\" ENOMEM in kernel/seccomp.c::seccomp_attach_filter() in 3.18 sources

The total length of all filter programs attached

to the calling thread would exceed

.B MAX_INSNS_PER_PATH

(32768) instructions.

Note that for the purposes of calculating this limit,

each already existing filter program incurs an

overhead penalty of 4 instructions.

.TP

.BR EOPNOTSUPP

.I operation

specified

.BR SECCOMP_GET_ACTION_AVAIL ,

but the kernel does not support the filter return action specified by

.IR args .

.TP

.BR ESRCH

Another thread caused a failure during thread sync, but its ID could not

be determined.

.SH VERSIONS

The

.BR seccomp ()

system call first appeared in Linux 3.17.

.\" FIXME . Add glibc version

.SH CONFORMING TO

The

.BR seccomp ()

system call is a nonstandard Linux extension.

.SH NOTES

Rather than hand-coding seccomp filters as shown in the example below,

you may prefer to employ the

.I libseccomp

library, which provides a front-end for generating seccomp filters.

.PP

The

.IR Seccomp

field of the

.IR /proc/[pid]/status

file provides a method of viewing the seccomp mode of a process; see

.BR proc (5).

.PP

.BR seccomp ()

provides a superset of the functionality provided by the

.BR prctl (2)

.BR PR_SET_SECCOMP

operation (which does not support

.IR flags ).

.PP

Since Linux 4.4, the

.BR prctl (2)

.B PTRACE_SECCOMP_GET_FILTER

operation can be used to dump a process's seccomp filters.

.\"

.SS Caveats

There are various subtleties to consider when applying seccomp filters

to a program, including the following:

.IP * 3

Some traditional system calls have user-space implementations in the

.BR vdso (7)

on many architectures.

Notable examples include

.BR clock_gettime (2),

.BR gettimeofday (2),

and

.BR time (2).

On such architectures,

seccomp filtering for these system calls will have no effect.

(However, there are cases where the

.BR vdso (7)

implementations may fall back to invoking the true system call,

in which case seccomp filters would see the system call.)

.IP *

Seccomp filtering is based on system call numbers.

However, applications typically do not directly invoke system calls,

but instead call wrapper functions in the C library which

in turn invoke the system calls.

Consequently, one must be aware of the following:

.RS

.IP \(bu 3

The glibc wrappers for some traditional system calls may actually

employ system calls with different names in the kernel.

For example, the

.BR exit (2)

wrapper function actually employs the

.BR exit_group (2)

system call, and the

.BR fork (2)

wrapper function actually calls

.BR clone (2).

.IP \(bu

The behavior of wrapper functions may vary across architectures,

according to the range of system calls provided on those architectures.

In other words, the same wrapper function may invoke

different system calls on different architectures.

.IP \(bu

Finally, the behavior of wrapper functions can change across glibc versions.

For example, in older versions, the glibc wrapper function for

.BR open (2)

invoked the system call of the same name,

but starting in glibc 2.26, the implementation switched to calling

.BR openat (2)

on all architectures.

.RE

.PP

The consequence of the above points is that it may be necessary

to filter for a system call other than might be expected.

Various manual pages in Section 2 provide helpful details

about the differences between wrapper functions and

the underlying system calls in subsections entitled

.IR "C library/kernel differences" .

.PP

Furthermore, note that the application of seccomp filters

even risks causing bugs in an application,

when the filters cause unexpected failures for legitimate operations

that the application might need to perform.

Such bugs may not easily be discovered when testing the seccomp

filters if the bugs occur in rarely used application code paths.

.RS 3

.\"

.SS Seccomp-specific BPF details

Note the following BPF details specific to seccomp filters:

.IP * 3

The

.B BPF_H

and

.B BPF_B

size modifiers are not supported: all operations must load and store

(4-byte) words

.RB ( BPF_W ).

.IP *

To access the contents of the

.I seccomp_data

buffer, use the

.B BPF_ABS

addressing mode modifier.

.IP *

The

.B BPF_LEN

addressing mode modifier yields an immediate mode operand

whose value is the size of the

.IR seccomp_data

buffer.

.SH EXAMPLE

The program below accepts four or more arguments.

The first three arguments are a system call number,

a numeric architecture identifier, and an error number.

The program uses these values to construct a BPF filter

that is used at run time to perform the following checks:

.IP [1] 4

If the program is not running on the specified architecture,

the BPF filter causes system calls to fail with the error

.BR ENOSYS .

.IP [2]

If the program attempts to execute the system call with the specified number,