Re: [PATCH] selftests/landlock: Fix snprintf truncation checks in test files

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