[PATCH v3] fstat: add test and documentation for an edge case.

[Adhemerval Zanella Netto]

On 14/06/25 08:25, Matteo Croce wrote:
> From: Matteo Croce <teknoraver@meta.com>
> 
> The fstatat behaviour when the target is a dangling symlink is different
> if flags contains AT_SYMLINK_NOFOLLOW or not.
> Add a test for this and document it.
> ---
>  io/tst-stat.c       | 16 +++++++++++++++-
>  manual/filesys.texi |  5 +++--
>  2 files changed, 18 insertions(+), 3 deletions(-)
> 
> diff --git a/io/tst-stat.c b/io/tst-stat.c
> index 61bdbfb638..56c7bbb186 100644
> --- a/io/tst-stat.c
> +++ b/io/tst-stat.c
> @@ -62,12 +62,22 @@ fstatat_check (int fd, const char *path, struct stat *st)
>    TEST_COMPARE (fstatat (fd, path, st, 0), 0);
>  }
>  
> +static void
> +fstatat_link (const char *path, struct stat *st)
> +{
> +  TEST_COMPARE (fstatat (AT_FDCWD, path, st, 0), -1);
> +  TEST_COMPARE (errno, ENOENT);
> +
> +  TEST_COMPARE (fstatat (AT_FDCWD, path, st, AT_SYMLINK_NOFOLLOW), 0);

Maybe also add a check if the result is the one expected?

The rest look ok to me, thanks for working on this.

> +}
> +
>  typedef void (*test_t)(int, const char *path, struct stat *);
>  
>  static int
>  do_test (void)
>  {
>    char *path;
> +  const char *linkame = "tst-fstat.linkname";
>    int fd = create_temp_file ("tst-fstat.", &path);
>    TEST_VERIFY_EXIT (fd >= 0);
>    support_write_file_string (path, "abc");
> @@ -81,13 +91,13 @@ do_test (void)
>      printf ("warning: timestamp with nanoseconds not supported\n");
>  
>    struct statx stx;
> +  struct stat st;
>    TEST_COMPARE (statx (fd, path, 0, STATX_BASIC_STATS, &stx), 0);
>  
>    test_t tests[] = { stat_check, lstat_check, fstat_check, fstatat_check };
>  
>    for (int i = 0; i < array_length (tests); i++)
>      {
> -      struct stat st;
>        tests[i](fd, path, &st);
>  
>        TEST_COMPARE (stx.stx_dev_major, major (st.st_dev));
> @@ -111,6 +121,10 @@ do_test (void)
>  	}
>      }
>  
> +  symlink ("tst-fstat.target", linkame);
> +  add_temp_file (linkame);
> +  fstatat_link (linkame, &st);
> +
>    return 0;
>  }
>  
> diff --git a/manual/filesys.texi b/manual/filesys.texi
> index d0ced84bf6..4406201dab 100644
> --- a/manual/filesys.texi
> +++ b/manual/filesys.texi
> @@ -2397,8 +2397,9 @@ The descriptor @var{filedes} is not associated with a directory, and
>  @var{filename} is a relative file name.
>  
>  @item ENOENT
> -The file named by @var{filename} does not exist, or @var{filename} is an
> -empty string and @var{flags} does not contain @code{AT_EMPTY_PATH}.
> +The file named by @var{filename} does not exist, it's a dangling symbolic link
> +and @var{flags} does not contain @code{AT_SYMLINK_NOFOLLOW}, or @var{filename}
> +is an empty string and @var{flags} does not contain @code{AT_EMPTY_PATH}.
>  @end table
>  
>  When the sources are compiled with @code{_FILE_OFFSET_BITS == 64} this