[PATCH 5/5] stdlib: Remove lstat usage from realpath [BZ #24970]

[Adhemerval Zanella]

On 28/12/2020 08:42, Adhemerval Zanella wrote:
>>>   static idx_t
>>> +readlink_scratch_buffer (const char *path, struct scratch_buffer *buf)
>>> +{
>>> +  ssize_t r;
>>> +  while (true)
>>> +    {
>>> +      ptrdiff_t bufsize = buf->length;
>>> +      r = __readlink (path, buf->data, bufsize - 1);
>>> +      if (r < bufsize - 1)
>>> +    break;
>>> +      if (!scratch_buffer_grow (buf))
>>> +    return -1;
>>> +    }
>>> +  return r;
>>> +}
>>
>> This function seems to exist because the proposed code calls readlink in two places. Current gnulib has been changed to call it in just one place, so there's less need to split out the function (the splitting complicates out-of-memory checking).
> 
> Yes, this trades a stat call by an extra readlink call.  However it seems
> that your strategy to use faccessat should be better, assuming it is lighter
> syscall.

Also, it would require a recent kernel (5.8) to avoid faccessat to fallback
to use __NR_faccessat (it would make each call to faccessat2 to issue
two syscall).  We can live with this and if it turns a performance issue we
ca the hack to set a global variable to avoid it.

Now this snippet:

 47 # include <sysdep.h>
 48 # ifdef __ASSUME_FACCESSAT2
 49 #  define FACCESSAT_NEVER_EOVERFLOWS __ASSUME_FACCESSAT2
 50 # else
 51 #  define FACCESSAT_NEVER_EOVERFLOWS true
 52 # endif

is not required, since Linux faccessat fallback uses LFS stat call that
does not return EOVERFLOW.  And it also bleeds Linux implementation details
on generic code.

I will send a patch to fix it.