On Fri, Jun 26, 2026 at 03:00:16PM +0800, Wang Yan wrote:
> Commit b566f7a4f0e4 ("selftests/landlock: Fix snprintf truncation checks
> in audit helpers") fixed the truncation detection in audit.h by changing
> the comparison from ">" to ">=" to correctly handle the edge case where
> snprintf returns a value equal to the buffer size.
>
> However, the same pattern exists in ptrace_test.c, audit_test.c, and
> net_test.c and was not fixed. snprintf() returns the number of characters
> that would have been written, excluding the terminating NUL byte. When
> the output is truncated, this return value equals or exceeds the buffer
> size. The existing ">" check therefore fails to detect truncation when
> the return value equals the buffer size.
>
> Fix these remaining instances to use ">=" for truncation detection,
> matching the fix in audit.h.
>
> Fixes: 6a500b22971c ("selftests/landlock: Add tests for audit flags and
> domain IDs")
> Signed-off-by: Haofeng Li <[email protected]>
> Signed-off-by: Wang Yan <[email protected]>
> ---
> tools/testing/selftests/landlock/audit_test.c | 2 +-
> tools/testing/selftests/landlock/net_test.c | 4 ++--
> tools/testing/selftests/landlock/ptrace_test.c | 2 +-
> 3 files changed, 4 insertions(+), 4 deletions(-)
>
> diff --git a/tools/testing/selftests/landlock/audit_test.c
> b/tools/testing/selftests/landlock/audit_test.c
> index 72b5612375dd..4bea8c880a4d 100644
> --- a/tools/testing/selftests/landlock/audit_test.c
> +++ b/tools/testing/selftests/landlock/audit_test.c
> @@ -31,7 +31,7 @@ static int matches_log_signal(struct __test_metadata *const
> _metadata,
>
> log_match_len =
> snprintf(log_match, sizeof(log_match), log_template, opid);
> - if (log_match_len > sizeof(log_match))
> + if (log_match_len >= sizeof(log_match))
> return -E2BIG;
>
> return audit_match_record(audit_fd, AUDIT_LANDLOCK_ACCESS, log_match,
> diff --git a/tools/testing/selftests/landlock/net_test.c
> b/tools/testing/selftests/landlock/net_test.c
> index 2ed1f76b7a8b..aebeafd80466 100644
> --- a/tools/testing/selftests/landlock/net_test.c
> +++ b/tools/testing/selftests/landlock/net_test.c
> @@ -2777,7 +2777,7 @@ static int matches_auditlog(const int audit_fd, const
> char *const blockers,
> log_match_len = snprintf(log_match, sizeof(log_match),
> log_with_addrport_tmpl, blockers,
> dir_addr, addr, dir_port, port);
> - if (log_match_len > sizeof(log_match))
> + if (log_match_len >= sizeof(log_match))
> return -E2BIG;
>
> return audit_match_record(audit_fd, AUDIT_LANDLOCK_ACCESS, log_match,
> @@ -3072,7 +3072,7 @@ static int matches_log_connect_bound(int audit_fd,
> const char *const blockers,
>
> log_match_len = snprintf(log_match, sizeof(log_match), log_template,
> blockers, addr, lport, addr, dport);
> - if (log_match_len > sizeof(log_match))
> + if (log_match_len >= sizeof(log_match))
> return -E2BIG;
>
> return audit_match_record(audit_fd, AUDIT_LANDLOCK_ACCESS, log_match,
> diff --git a/tools/testing/selftests/landlock/ptrace_test.c
> b/tools/testing/selftests/landlock/ptrace_test.c
> index 4f64c90583cd..65cf2d82f721 100644
> --- a/tools/testing/selftests/landlock/ptrace_test.c
> +++ b/tools/testing/selftests/landlock/ptrace_test.c
> @@ -302,7 +302,7 @@ static int matches_log_ptrace(struct __test_metadata
> *const _metadata,
>
> log_match_len =
> snprintf(log_match, sizeof(log_match), log_template, opid);
> - if (log_match_len > sizeof(log_match))
> + if (log_match_len >= sizeof(log_match))
> return -E2BIG;
>
> return audit_match_record(audit_fd, AUDIT_LANDLOCK_ACCESS, log_match,
> --
> 2.25.1
>
Thanks for the fix and the reminder! The fix is correct, because snprintf()
returns the number of bytes that would have been written *excluding* the
trailing NUL byte, whereas the buffer size passed to it *includes* the
space for the trailing NUL byte.
Reviewed-by: Günther Noack <[email protected]>
—Günther
