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, ...);

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).

The SCMP_CMP() and SCMP_A{0-5}() macros generate a scmp_arg_cmp structure for use with the above functions. The SCMP_CMP() macro allows the caller to specify an arbitrary argument along with the comparison operator, mask, and datum values where the SCMP_A{0-5}() macros are specific to a certain argument. See the EXAMPLES section below.

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.

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_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_ALLOW: The seccomp filter will have no effect on the thread calling the syscall if it matches the filter rule.

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 seccomp_rule_add(), seccomp_rule_add_array(), seccomp_rule_add_exact(), and seccomp_rule_add_exact_array() functions return zero on success, negative errno values on failure.

EXAMPLES

#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;
}

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.

AUTHOR

Paul Moore <paul@paul-moore.com>