Provided by: libseccomp-dev_2.5.5-1ubuntu6_amd64 
NAME
seccomp_rule_add, seccomp_rule_add_exact - Add a seccomp filter rule
SYNOPSIS
#include <seccomp.h>
typedef void * scmp_filter_ctx;
int SCMP_SYS(syscall_name);
struct scmp_arg_cmp SCMP_CMP(unsigned int arg,
enum scmp_compare op, ...);
struct scmp_arg_cmp SCMP_A0(enum scmp_compare op, ...);
struct scmp_arg_cmp SCMP_A1(enum scmp_compare op, ...);
struct scmp_arg_cmp SCMP_A2(enum scmp_compare op, ...);
struct scmp_arg_cmp SCMP_A3(enum scmp_compare op, ...);
struct scmp_arg_cmp SCMP_A4(enum scmp_compare op, ...);
struct scmp_arg_cmp SCMP_A5(enum scmp_compare op, ...);
struct scmp_arg_cmp SCMP_CMP64(unsigned int arg,
enum scmp_compare op, ...);
struct scmp_arg_cmp SCMP_A0_64(enum scmp_compare op, ...);
struct scmp_arg_cmp SCMP_A1_64(enum scmp_compare op, ...);
struct scmp_arg_cmp SCMP_A2_64(enum scmp_compare op, ...);
struct scmp_arg_cmp SCMP_A3_64(enum scmp_compare op, ...);
struct scmp_arg_cmp SCMP_A4_64(enum scmp_compare op, ...);
struct scmp_arg_cmp SCMP_A5_64(enum scmp_compare op, ...);
struct scmp_arg_cmp SCMP_CMP32(unsigned int arg,
enum scmp_compare op, ...);
struct scmp_arg_cmp SCMP_A0_32(enum scmp_compare op, ...);
struct scmp_arg_cmp SCMP_A1_32(enum scmp_compare op, ...);
struct scmp_arg_cmp SCMP_A2_32(enum scmp_compare op, ...);
struct scmp_arg_cmp SCMP_A3_32(enum scmp_compare op, ...);
struct scmp_arg_cmp SCMP_A4_32(enum scmp_compare op, ...);
struct scmp_arg_cmp SCMP_A5_32(enum scmp_compare op, ...);
int seccomp_rule_add(scmp_filter_ctx ctx, uint32_t action,
int syscall, unsigned int arg_cnt, ...);
int seccomp_rule_add_exact(scmp_filter_ctx ctx, uint32_t action,
int syscall, unsigned int arg_cnt, ...);
int seccomp_rule_add_array(scmp_filter_ctx ctx,
uint32_t action, int syscall,
unsigned int arg_cnt,
const struct scmp_arg_cmp *arg_array);
int seccomp_rule_add_exact_array(scmp_filter_ctx ctx,
uint32_t action, int syscall,
unsigned int arg_cnt,
const struct scmp_arg_cmp *arg_array);
Link with -lseccomp.
DESCRIPTION
The seccomp_rule_add(), seccomp_rule_add_array(), seccomp_rule_add_exact(), and
seccomp_rule_add_exact_array() functions all add a new filter rule to the current seccomp filter. The
seccomp_rule_add() and 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 seccomp_rule_add_exact() and
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,
seccomp_rule_add() and seccomp_rule_add_array() do guarantee the same behavior regardless of the
architecture.
The newly added filter rule does not take effect until the entire filter is loaded into the kernel using
seccomp_load(3). When adding rules to a filter, it is important to consider the impact of previously
loaded filters; see the seccomp_load(3) documentation for more information.
All of the filter rules supplied by the calling application are combined into a union, with additional
logic to eliminate redundant syscall filters. For example, if a rule is added which allows a given
syscall with a specific set of argument values and later a rule is added which allows the same syscall
regardless the argument values then the first, more specific rule, is effectively dropped from the filter
by the second more generic rule.
The SCMP_CMP(), SCMP_CMP64(), SCMP_A{0-5}(), and SCMP_A{0-5}_64() macros generate a scmp_arg_cmp
structure for use with the above functions. The SCMP_CMP() and SCMP_CMP64() macros allows the caller to
specify an arbitrary argument along with the comparison operator, 64-bit mask, and 64-bit datum values
where the SCMP_A{0-5}() and SCMP_A{0-5}_64() macros are specific to a certain argument.
The SCMP_CMP32() and SCMP_A{0-5}_32() macros are similar to the variants above, but they take 32-bit mask
and 32-bit datum values.
It is recommended that whenever possible developers avoid using the SCMP_CMP() and SCMP_A{0-5}() macros
and use the variants which are explicitly 32 or 64-bit. This should help eliminate problems caused by an
unwanted sign extension of negative datum values.
If syscall argument comparisons are included in the filter rule, all of the comparisons must be true for
the rule to match.
When adding syscall argument comparisons to the filter it is important to remember that while it is
possible to have multiple comparisons in a single rule, you can only compare each argument once in a
single rule. In other words, you can not have multiple comparisons of the 3rd syscall argument in a
single rule.
In a filter containing multiple architectures, it is an error to add a filter rule for a syscall that
does not exist in all of the filter's architectures.
While it is possible to specify the syscall value directly using the standard __NR_syscall values, in
order to ensure proper operation across multiple architectures it is highly recommended to use the
SCMP_SYS() macro instead. See the EXAMPLES section below. It is also important to remember that
regardless of the architectures present in the filter, the syscall numbers used in filter rules are
interpreted in the context of the native architecture.
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 SCMP_FLTATR_API_TSKIP attribute. See seccomp_attr_set(3) for more
information.
The filter context ctx is the value returned by the call to seccomp_init(3).
Valid action values are as follows:
SCMP_ACT_KILL
The thread will be killed by the kernel when it calls a syscall that matches the filter rule.
SCMP_ACT_KILL_PROCESS
The process will be killed by the kernel when it calls a syscall that matches the filter rule.
SCMP_ACT_TRAP
The thread will throw a SIGSYS signal when it calls a syscall that matches the filter rule.
SCMP_ACT_ERRNO(uint16_t errno)
The thread will receive a return value of errno when it calls a syscall that matches the filter
rule.
SCMP_ACT_TRACE(uint16_t msg_num)
If the thread is being traced and the tracing process specified the PTRACE_O_TRACESECCOMP option
in the call to ptrace(2), the tracing process will be notified, via PTRACE_EVENT_SECCOMP , and the
value provided in msg_num can be retrieved using the PTRACE_GETEVENTMSG option.
SCMP_ACT_LOG
The seccomp filter will have no effect on the thread calling the syscall if it matches the filter
rule but the syscall will be logged.
SCMP_ACT_ALLOW
The seccomp filter will have no effect on the thread calling the syscall if it matches the filter
rule.
SCMP_ACT_NOTIFY
A monitoring process will be notified when a process running the seccomp filter calls a syscall
that matches the filter rule. The process that invokes the syscall waits in the kernel until the
monitoring process has responded via seccomp_notify_respond (3) .
When a filter utilizing SCMP_ACT_NOTIFY is loaded into the kernel, the kernel generates a
notification fd that must be used to communicate between the monitoring process and the
process(es) being filtered. See seccomp_notif_fd (3) for more information.
Valid comparison op values are as follows:
SCMP_CMP_NE
Matches when the argument value is not equal to the datum value, example:
SCMP_CMP( arg , SCMP_CMP_NE , datum )
SCMP_CMP_LT
Matches when the argument value is less than the datum value, example:
SCMP_CMP( arg , SCMP_CMP_LT , datum )
SCMP_CMP_LE
Matches when the argument value is less than or equal to the datum value, example:
SCMP_CMP( arg , SCMP_CMP_LE , datum )
SCMP_CMP_EQ
Matches when the argument value is equal to the datum value, example:
SCMP_CMP( arg , SCMP_CMP_EQ , datum )
SCMP_CMP_GE
Matches when the argument value is greater than or equal to the datum value, example:
SCMP_CMP( arg , SCMP_CMP_GE , datum )
SCMP_CMP_GT
Matches when the argument value is greater than the datum value, example:
SCMP_CMP( arg , SCMP_CMP_GT , datum )
SCMP_CMP_MASKED_EQ
Matches when the masked argument value is equal to the masked datum value, example:
SCMP_CMP( arg , SCMP_CMP_MASKED_EQ , mask , datum )
RETURN VALUE
The SCMP_SYS() macro returns a value suitable for use as the syscall value in the seccomp_rule_add*()
functions. In a similar manner, the SCMP_CMP() and SCMP_A*() macros return values suitable for use as
argument comparisons in the seccomp_rule_add() and seccomp_rule_add_exact() functions.
The seccomp_rule_add(), seccomp_rule_add_array(), seccomp_rule_add_exact(), and
seccomp_rule_add_exact_array() functions return zero on success or one of the following error codes on
failure:
-EDOM Architecture specific failure.
-EEXIST
The rule already exists.
-EACCCES
The rule conflicts with the filter (for example, the rule action equals the default action of the
filter).
-EFAULT
Internal libseccomp failure.
-EINVAL
Invalid input, either the context or architecture token is invalid.
-ENOMEM
The library was unable to allocate enough memory.
-EOPNOTSUPP
The library doesn't support the particular operation.
EXAMPLES
#include <fcntl.h>
#include <seccomp.h>
#include <sys/stat.h>
#include <sys/types.h>
#include <stddef.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(exit_group), 0);
if (rc < 0)
goto out;
rc = seccomp_rule_add(ctx, SCMP_ACT_ALLOW, SCMP_SYS(exit), 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;
}
NOTES
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.
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.
BUGS
The runtime behavior of seccomp filters is dependent upon the kernel version, the processor architecture,
and other libraries including libc. This could affect the return code of a seccomp filter.
* PowerPC glibc will not return a negative number when the getpid() syscall is invoked. If a
seccomp filter has been created where getpid() will return a negative number from the kernel, then
PowerPC glibc will return the absolute value of the errno. In this case, it is very difficult for
an application to distinguish between the errno and a valid pid.
AUTHOR
Paul Moore <paul@paul-moore.com>
SEE ALSO
seccomp_syscall_resolve_name_rewrite(3), seccomp_syscall_priority(3), seccomp_load(3),
seccomp_attr_set(3)
paul@paul-moore.com 30 May 2020 seccomp_rule_add(3)