Re: [PATCH 1/3] stdlib: Use fixed buffer size for realpath (BZ #26241)

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

On 12/08/2020 20:04, Adhemerval Zanella wrote:
> 
> 
> On 11/08/2020 22:38, Xiaoming Ni wrote:

> With something like below we can make the realpath uses a stack of about
> ~1024 and ~2048 if the path contains symbolic link:
> 


> @@ -163,35 +204,19 @@ __realpath (const char *name, char *resolved)
>  
>  	  if (S_ISLNK (st.st_mode))
>  	    {
> -	      char *buf = __alloca (path_max);
> -	      size_t len;
> -
>  	      if (++num_links > __eloop_threshold ())
>  		{
>  		  __set_errno (ELOOP);
>  		  goto error;
>  		}
>  
> -	      n = __readlink (rpath, buf, path_max - 1);
> -	      if (n < 0)
> +	      if (! realpath_readlink (rpath, end, path_max, st.st_size,
> +				       &extra_buf))

Scratch that, unfortunately some Linux filesystems do not return the symlink
target file size with a lstat call (procfs and sysfs for instance).  So
we need to either issue multiple readlink with different increasing buffers
or assume PATH_MAX (as current algorithms does).

I will send a v3 of my patch to use a scratch buffer.  What I am not sure is
if the Linux optimization is worth if the idea is use a small stack as possible
for common cases since it trades some syscall by a higher stack usage.