Re: [RFC PATCH 1/2] elf: Port ldconfig away from stack-allocated paths

[Adhemerval Zanella Netto]

On 20/05/23 16:03, Sergey Bugaev wrote:
> Hello,
> 
> On Fri, May 19, 2023 at 3:25 PM Florian Weimer <fweimer@redhat.com> wrote:
>> Missing error checking?
> 
> Indeed, thanks. Fixed.
> 
>> I think we use x* functions such a xstrdup elsewhere in ldconfig, so you
>> could add xasprintf as well.
> 
> Apparently the existing code didn't #include <support/support.h> and link
> to libsupport (xstrdup must be coming from somewhere else.) I opted to
> check for asprintf returning < 0 and calling error () explicitly instead,
> like other code in this same file does already.
> 
> Sergey
> 
> -- >8 --
> Subject: [PATCH 1/2 v2] elf: Port ldconfig away from stack-allocated paths
> 
> ldconfig was allocating PATH_MAX bytes on the stack for the library file
> name. The issues with PATH_MAX usage are well documented [0][1]; even if
> a program does not rely on paths being limited to PATH_MAX bytes,
> allocating 4096 bytes on the stack for paths that are typically rather
> short (strlen ("/lib64/libc.so.6") is 16) is wasteful and dangerous.
> 
> [0]: https://insanecoding.blogspot.com/2007/11/pathmax-simply-isnt.html
> [1]: https://eklitzke.org/path-max-is-tricky
> 
> Instead, make use of asprintf to dynamically allocate memory of just the
> right size on the heap.
> 
> Reviewed-by: Adhemerval Zanella <adhemerval.zanella@linaro.org>
> Signed-off-by: Sergey Bugaev <bugaevc@gmail.com>

Still looks good to me, thanks.

> ---
>  elf/ldconfig.c | 60 +++++++++++++++++++-------------------------------
>  1 file changed, 23 insertions(+), 37 deletions(-)
> 
> diff --git a/elf/ldconfig.c b/elf/ldconfig.c
> index 2fc45ad8..e643dd57 100644
> --- a/elf/ldconfig.c
> +++ b/elf/ldconfig.c
> @@ -677,28 +677,18 @@ search_dir (const struct dir_entry *entry)
>  
>    char *dir_name;
>    char *real_file_name;
> -  size_t real_file_name_len;
> -  size_t file_name_len = PATH_MAX;
> -  char *file_name = alloca (file_name_len);
> +  char *file_name;
>    if (opt_chroot != NULL)
> -    {
> -      dir_name = chroot_canon (opt_chroot, entry->path);
> -      real_file_name_len = PATH_MAX;
> -      real_file_name = alloca (real_file_name_len);
> -    }
> +    dir_name = chroot_canon (opt_chroot, entry->path);
>    else
> -    {
> -      dir_name = entry->path;
> -      real_file_name_len = 0;
> -      real_file_name = file_name;
> -    }
> +    dir_name = entry->path;
>  
>    DIR *dir;
>    if (dir_name == NULL || (dir = opendir (dir_name)) == NULL)
>      {
>        if (opt_verbose)
>       error (0, errno, _("Can't open directory %s"), entry->path);
> -      if (opt_chroot != NULL && dir_name != NULL)
> +      if (opt_chroot != NULL)
>       free (dir_name);
>        return;
>      }
> @@ -733,25 +723,16 @@ search_dir (const struct dir_entry *entry)
>                        + 1, ".#prelink#.", sizeof (".#prelink#.") - 1) == 0)
>           continue;
>       }
> -      len += strlen (entry->path) + 2;
> -      if (len > file_name_len)
> -     {
> -       file_name_len = len;
> -       file_name = alloca (file_name_len);
> -       if (!opt_chroot)
> -         real_file_name = file_name;
> -     }
> -      sprintf (file_name, "%s/%s", entry->path, direntry->d_name);
> +      if (asprintf (&file_name, "%s/%s", entry->path, direntry->d_name) < 0)
> +     error (EXIT_FAILURE, errno, _("Could not form library path"));
>        if (opt_chroot != NULL)
>       {
> -       len = strlen (dir_name) + strlen (direntry->d_name) + 2;
> -       if (len > real_file_name_len)
> -         {
> -           real_file_name_len = len;
> -           real_file_name = alloca (real_file_name_len);
> -         }
> -       sprintf (real_file_name, "%s/%s", dir_name, direntry->d_name);
> +       if (asprintf (&real_file_name, "%s/%s",
> +                     dir_name, direntry->d_name) < 0)
> +         error (EXIT_FAILURE, errno, _("Could not form library path"));
>       }
> +      else
> +     real_file_name = file_name;
>  
>        struct stat lstat_buf;
>        /* We optimize and try to do the lstat call only if needed.  */
> @@ -761,7 +742,7 @@ search_dir (const struct dir_entry *entry)
>       if (__glibc_unlikely (lstat (real_file_name, &lstat_buf)))
>         {
>           error (0, errno, _("Cannot lstat %s"), file_name);
> -         continue;
> +         goto next;
>         }
>  
>        struct stat stat_buf;
> @@ -778,7 +759,7 @@ search_dir (const struct dir_entry *entry)
>               {
>                 if (strstr (file_name, ".so") == NULL)
>                   error (0, 0, _("Input file %s not found.\n"), file_name);
> -               continue;
> +               goto next;
>               }
>           }
>         if (__glibc_unlikely (stat (target_name, &stat_buf)))
> @@ -793,7 +774,7 @@ search_dir (const struct dir_entry *entry)
>             if (opt_chroot != NULL)
>               free (target_name);
>  
> -           continue;
> +           goto next;
>           }
>  
>         if (opt_chroot != NULL)
> @@ -806,7 +787,7 @@ search_dir (const struct dir_entry *entry)
>         lstat_buf.st_ctime = stat_buf.st_ctime;
>       }
>        else if (!S_ISREG (lstat_buf.st_mode))
> -     continue;
> +     goto next;
>  
>        char *real_name;
>        if (opt_chroot != NULL && is_link)
> @@ -816,7 +797,7 @@ search_dir (const struct dir_entry *entry)
>           {
>             if (strstr (file_name, ".so") == NULL)
>               error (0, 0, _("Input file %s not found.\n"), file_name);
> -           continue;
> +           goto next;
>           }
>       }
>        else
> @@ -828,7 +809,7 @@ search_dir (const struct dir_entry *entry)
>         && __builtin_expect (lstat (real_file_name, &lstat_buf), 0))
>       {
>         error (0, errno, _("Cannot lstat %s"), file_name);
> -       continue;
> +       goto next;
>       }
>  
>        /* First search whether the auxiliary cache contains this
> @@ -842,7 +823,7 @@ search_dir (const struct dir_entry *entry)
>           {
>             if (real_name != real_file_name)
>               free (real_name);
> -           continue;
> +           goto next;
>           }
>         else if (opt_build_cache)
>           add_to_aux_cache (&lstat_buf, flag, isa_level, soname);
> @@ -948,6 +929,11 @@ search_dir (const struct dir_entry *entry)
>         dlib_ptr->next = dlibs;
>         dlibs = dlib_ptr;
>       }
> +
> +    next:
> +      free (file_name);
> +      if (opt_chroot != NULL)
> +     free (real_file_name);
>      }
>  
>    closedir (dir);