Re: [PATCH 4/4] stdlib: Remove lstat usage from realpath [BZ #24970]

[Adhemerval Zanella <adhemerval.zanella@linaro.org>]

Ping.

On 10/09/2020 12:19, Adhemerval Zanella wrote:
> The readlink already tells whether the file is a symlink, so there is
> no need to call lstat to check it.  However for '..' it requires an
> extra readlink check if the previous component can be really accessed,
> otherwise the next iteration will check a possible valid path and end
> early.  It should performance-wise acceptable and a gain over lstat,
> afaik symlink should not update any inode information.
> 
> Checked on x86_64-linux-gnu and i686-linux-gnu.
> ---
>  stdlib/canonicalize.c | 52 ++++++++++++++++++++++---------------------
>  1 file changed, 27 insertions(+), 25 deletions(-)
> 
> diff --git a/stdlib/canonicalize.c b/stdlib/canonicalize.c
> index 44a25a9a59..952f4dca41 100644
> --- a/stdlib/canonicalize.c
> +++ b/stdlib/canonicalize.c
> @@ -55,6 +55,7 @@ __realpath (const char *name, char *resolved)
>    const char *start, *end, *rpath_limit;
>    const size_t path_max = PATH_MAX;
>    int num_links = 0;
> +  char buf[PATH_MAX];
>    char extra_buf[PATH_MAX];
>  
>    if (name == NULL)
> @@ -104,12 +105,6 @@ __realpath (const char *name, char *resolved)
>  
>    for (end = start; *start; start = end)
>      {
> -#ifdef _LIBC
> -      struct stat64 st;
> -#else
> -      struct stat st;
> -#endif
> -
>        /* Skip sequence of multiple path-separators.  */
>        while (*start == '/')
>          ++start;
> @@ -118,12 +113,25 @@ __realpath (const char *name, char *resolved)
>        for (end = start; *end && *end != '/'; ++end)
>          /* Nothing.  */;
>  
> -      if (end - start == 0)
> -        break;
> -      else if (end - start == 1 && start[0] == '.')
> +      if (end - start == 1 && start[0] == '.')
>          /* nothing */;
>        else if (end - start == 2 && start[0] == '.' && start[1] == '.')
>          {
> +          ssize_t n;
> +
> +          if (dest[-1] != '/')
> +            *dest++ = '/';
> +          *dest = '\0';
> +
> +          n = __readlink (rpath, buf, path_max - 1);
> +          if (n == -1)
> +            {
> +              if (errno == ENOTDIR && dest[-1] == '/')
> +                dest[-1] = '\0';
> +              if (errno != EINVAL)
> +                goto error;
> +            }
> +
>            /* Back up to previous component, ignore if at root already.  */
>            if (dest > rpath + 1)
>              for (--dest; dest > rpath && dest[-1] != '/'; --dest)
> @@ -132,6 +140,7 @@ __realpath (const char *name, char *resolved)
>        else
>          {
>            size_t new_size;
> +          ssize_t n;
>  
>            if (dest[-1] != '/')
>              *dest++ = '/';
> @@ -166,25 +175,23 @@ __realpath (const char *name, char *resolved)
>            dest = __mempcpy (dest, start, end - start);
>            *dest = '\0';
>  
> -          if (__lxstat64 (_STAT_VER, rpath, &st) < 0)
> -            goto error;
> -
> -          if (S_ISLNK (st.st_mode))
> +          n = __readlink (rpath, buf, path_max - 1);
> +          if (n < 0)
> +            {
> +              if (errno == ENOTDIR && dest[-1] == '/')
> +                dest[-1] = '\0';
> +              if (errno != EINVAL)
> +                goto error;
> +            }
> +          else
>              {
> -              char buf[PATH_MAX];
>                size_t len;
> -              ssize_t n;
>  
>                if (++num_links > __eloop_threshold ())
>                  {
>                    __set_errno (ELOOP);
>                    goto error;  }
>  
> -              n = __readlink (rpath, buf, path_max - 1);
> -              if (n < 0)
> -                goto error;
> -              buf[n] = '\0';
> -
>                len = strlen (end);
>                /* Check that n + len + 1 doesn't overflow and is <= path_max. */
>                if (n >= SIZE_MAX - len || n + len >= path_max)
> @@ -211,11 +218,6 @@ __realpath (const char *name, char *resolved)
>                        continue;
>                  }
>              }
> -          else if (!S_ISDIR (st.st_mode) && *end != '\0')
> -            {
> -              __set_errno (ENOTDIR);
> -              goto error;
> -            }
>          }
>      }
>    if (dest > rpath + 1 && dest[-1] == '/')
>