.TH "seccomp_rule_add" 3 "25 July 2012" "paul@paul-moore.com" "libseccomp Documentation"
.\" //////////////////////////////////////////////////////////////////////////
.SH NAME
.\" //////////////////////////////////////////////////////////////////////////
seccomp_rule_add, seccomp_rule_add_exact \- Add a seccomp filter rule
.\" //////////////////////////////////////////////////////////////////////////
.SH SYNOPSIS
.\" //////////////////////////////////////////////////////////////////////////
.nf
.B #include <seccomp.h>
.sp
.B typedef void * scmp_filter_ctx;
.sp
.BI "int SCMP_SYS(" syscall_name ");"
.sp
.BI "struct scmp_arg_cmp SCMP_CMP(unsigned int " arg ","
.BI "                             enum scmp_compare " op ", " ... ");"
.BI "struct scmp_arg_cmp SCMP_A0(enum scmp_compare " op ", " ... ");"
.BI "struct scmp_arg_cmp SCMP_A1(enum scmp_compare " op ", " ... ");"
.BI "struct scmp_arg_cmp SCMP_A2(enum scmp_compare " op ", " ... ");"
.BI "struct scmp_arg_cmp SCMP_A3(enum scmp_compare " op ", " ... ");"
.BI "struct scmp_arg_cmp SCMP_A4(enum scmp_compare " op ", " ... ");"
.BI "struct scmp_arg_cmp SCMP_A5(enum scmp_compare " op ", " ... ");"
.sp
.BI "int seccomp_rule_add(scmp_filter_ctx " ctx ", uint32_t " action ","
.BI "                     int " syscall ", unsigned int " arg_cnt ", " ... ");"
.BI "int seccomp_rule_add_exact(scmp_filter_ctx " ctx ", uint32_t " action ","
.BI "                           int " syscall ", unsigned int " arg_cnt ", " ... ");"
.sp
.BI "int seccomp_rule_add_array(scmp_filter_ctx " ctx ","
.BI "                           uint32_t " action ", int " syscall ","
.BI "                           unsigned int " arg_cnt ","
.BI "                           const struct scmp_arg_cmp *"arg_array ");"
.BI "int seccomp_rule_add_exact_array(scmp_filter_ctx " ctx ","
.BI "                                 uint32_t " action ", int " syscall ","
.BI "                                 unsigned int " arg_cnt ","
.BI "                                 const struct scmp_arg_cmp *"arg_array ");"
.sp
Link with \fI\-lseccomp\fP.
.fi
.\" //////////////////////////////////////////////////////////////////////////
.SH DESCRIPTION
.\" //////////////////////////////////////////////////////////////////////////
.P
The
.BR seccomp_rule_add (),
.BR seccomp_rule_add_array (),
.BR seccomp_rule_add_exact (),
and
.BR seccomp_rule_add_exact_array ()
functions all add a new filter rule to the current seccomp filter.  The
.BR seccomp_rule_add ()
and
.BR seccomp_rule_add_array ()
functions will make a "best effort" to add the rule as specified, but may alter
the rule slightly due to architecture specifics (e.g. internal rewriting of
multiplexed syscalls, like socket and ipc functions on x86).  The
.BR seccomp_rule_add_exact ()
and
.BR seccomp_rule_add_exact_array ()
functions will attempt to add the rule exactly as specified so it may behave
differently on different architectures.  While it does not guarantee a exact
filter ruleset,
.BR seccomp_rule_add ()
and
.BR seccomp_rule_add_array ()
do guarantee the same behavior regardless of the architecture.
.P
The newly added filter rule does not take effect until the entire filter is
loaded into the kernel using
.BR seccomp_load (3).
.P
The
.BR SCMP_CMP ()
and
.BR SCMP_A{0-5} ()
macros generate a scmp_arg_cmp structure for use with the above functions. The
.BR SCMP_CMP ()
macro allows the caller to specify an arbitrary argument along with the
comparison operator, mask, and datum values where the
.BR SCMP_A{0-5} ()
macros are specific to a certain argument.  See the EXAMPLES section below.
.P
While it is possible to specify the
.I syscall
value directly using the standard
.B __NR_syscall
values, in order to ensure proper operation across multiple architectures it
is highly recommended to use the
.BR SCMP_SYS ()
macro instead.  See the EXAMPLES section below.
.P
Starting with Linux v4.8, there may be a need to create a rule with a syscall
value of -1 to allow tracing programs to skip a syscall invocation; in order
to create a rule with a -1 syscall value it is necessary to first set the
.B SCMP_FLTATR_API_TSKIP
attribute.  See
.BR seccomp_attr_set (3)
for more information.
.P
The filter context
.I ctx
is the value returned by the call to
.BR seccomp_init (3).
.P
Valid
.I action
values are as follows:
.TP
.B SCMP_ACT_KILL
The thread will be killed by the kernel when it calls a syscall that matches
the filter rule.
.TP
.B SCMP_ACT_TRAP
The thread will throw a SIGSYS signal when it calls a syscall that matches the
filter rule.
.TP
.B SCMP_ACT_ERRNO(uint16_t errno)
The thread will receive a return value of
.I errno
when it calls a syscall that matches the filter rule.
.TP
.B SCMP_ACT_TRACE(uint16_t msg_num)
If the thread is being traced and the tracing process specified the
.B PTRACE_O_TRACESECCOMP
option in the call to
.BR ptrace (2),
the tracing process will be notified, via
.B PTRACE_EVENT_SECCOMP
, and the value provided in
.I msg_num
can be retrieved using the
.B PTRACE_GETEVENTMSG
option.
.TP
.B SCMP_ACT_ALLOW
The seccomp filter will have no effect on the thread calling the syscall if it
matches the filter rule.
.P
Valid comparison
.I op
values are as follows:
.TP
.B SCMP_CMP_NE
Matches when the argument value is not equal to the datum value, example:
.sp
SCMP_CMP(
.I arg
, SCMP_CMP_NE ,
.I datum
)
.TP
.B SCMP_CMP_LT
Matches when the argument value is less than the datum value, example:
.sp
SCMP_CMP(
.I arg
, SCMP_CMP_LT ,
.I datum
)
.TP
.B SCMP_CMP_LE
Matches when the argument value is less than or equal to the datum value,
example:
.sp
SCMP_CMP(
.I arg
, SCMP_CMP_LE ,
.I datum
)
.TP
.B SCMP_CMP_EQ
Matches when the argument value is equal to the datum value, example:
.sp
SCMP_CMP(
.I arg
, SCMP_CMP_EQ ,
.I datum
)
.TP
.B SCMP_CMP_GE
Matches when the argument value is greater than or equal to the datum value,
example:
.sp
SCMP_CMP(
.I arg
, SCMP_CMP_GE ,
.I datum
)
.TP
.B SCMP_CMP_GT
Matches when the argument value is greater than the datum value, example:
.sp
SCMP_CMP(
.I arg
, SCMP_CMP_GT ,
.I datum
)
.TP
.B SCMP_CMP_MASKED_EQ
Matches when the masked argument value is equal to the masked datum value,
example:
.sp
SCMP_CMP(
.I arg
, SCMP_CMP_MASKED_EQ ,
.I mask
,
.I datum
)
.\" //////////////////////////////////////////////////////////////////////////
.SH RETURN VALUE
.\" //////////////////////////////////////////////////////////////////////////
The
.BR seccomp_rule_add (),
.BR seccomp_rule_add_array (),
.BR seccomp_rule_add_exact (),
and
.BR seccomp_rule_add_exact_array ()
functions return zero on success, negative errno values on failure.
.\" //////////////////////////////////////////////////////////////////////////
.SH EXAMPLES
.\" //////////////////////////////////////////////////////////////////////////
.nf
#include <fcntl.h>
#include <seccomp.h>
#include <sys/stat.h>
#include <sys/types.h>

#define BUF_SIZE	256

int main(int argc, char *argv[])
{
	int rc = \-1;
	scmp_filter_ctx ctx;
	struct scmp_arg_cmp arg_cmp[] = { SCMP_A0(SCMP_CMP_EQ, 2) };
	int fd;
	unsigned char buf[BUF_SIZE];

	ctx = seccomp_init(SCMP_ACT_KILL);
	if (ctx == NULL)
		goto out;

	/* ... */

	fd = open("file.txt", 0);

	/* ... */

	rc = seccomp_rule_add(ctx, SCMP_ACT_ALLOW, SCMP_SYS(close), 0);
	if (rc < 0)
		goto out;

	rc = seccomp_rule_add(ctx, SCMP_ACT_ALLOW, SCMP_SYS(read), 3,
			      SCMP_A0(SCMP_CMP_EQ, fd),
			      SCMP_A1(SCMP_CMP_EQ, (scmp_datum_t)buf),
			      SCMP_A2(SCMP_CMP_LE, BUF_SIZE));
	if (rc < 0)
		goto out;

	rc = seccomp_rule_add(ctx, SCMP_ACT_ALLOW, SCMP_SYS(write), 1,
			      SCMP_CMP(0, SCMP_CMP_EQ, fd));
	if (rc < 0)
		goto out;

	rc = seccomp_rule_add_array(ctx, SCMP_ACT_ALLOW, SCMP_SYS(write), 1,
			            arg_cmp);
	if (rc < 0)
		goto out;

	rc = seccomp_load(ctx);
	if (rc < 0)
		goto out;

	/* ... */

out:
	seccomp_release(ctx);
	return \-rc;
}
.fi
.\" //////////////////////////////////////////////////////////////////////////
.SH NOTES
.\" //////////////////////////////////////////////////////////////////////////
.P
While the seccomp filter can be generated independent of the kernel, kernel
support is required to load and enforce the seccomp filter generated by
libseccomp.
.P
The libseccomp project site, with more information and the source code
repository, can be found at https://github.com/seccomp/libseccomp.  This tool,
as well as the libseccomp library, is currently under development, please
report any bugs at the project site or directly to the author.
.\" //////////////////////////////////////////////////////////////////////////
.SH AUTHOR
.\" //////////////////////////////////////////////////////////////////////////
Paul Moore <paul@paul-moore.com>
.\" //////////////////////////////////////////////////////////////////////////
.SH SEE ALSO
.\" //////////////////////////////////////////////////////////////////////////
.BR seccomp_syscall_resolve_name_rewrite (3),
.BR seccomp_syscall_priority (3),
.BR seccomp_load (3),
.BR seccomp_attr_set (3)