GNU bug report logs - #13530
head: memory exhausted when printing all from stdin but last P/E bytes

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Message #25 received at 13530-done <at> debbugs.gnu.org (full text, mbox):

From: Pádraig Brady <P <at> draigBrady.com>
To: Lei Zhang <lei.zhang <at> uwaterloo.ca>
Cc: 13530-done <at> debbugs.gnu.org
Subject: Re: bug#13530: head: memory exhausted when printing all from stdin
	but last P/E bytes
Date: Fri, 12 Apr 2013 19:18:49 +0100

[Message part 1 (text/plain, inline)]

On 01/23/2013 12:34 PM, Pádraig Brady wrote:
> On 01/23/2013 02:32 AM, Lei Zhang wrote:
>> Hi All,
>>
>> We found a bug in the `head' program of coreutils 8.20:
>>
>> Invoking `head -c -P' or `head -c -E' will cause memory exhaustion.
>>
>> However, smaller units (e.g., b, K, M) work fine; bigger units (e.g., Z, Y)
>> fail properly
>> (by outputing "number of bytes is so large that it is not representable").
>> And `-n' also
>> works fine.
>>
>> A bit dig reveals that the bug is introduced at line 322 of head.c
>> (coreutils 8.20):
>>
>> 204: elide_tail_bytes_pipe (const char *filename, int fd, uintmax_t
>> n_elide_0)
>> 205: {
>> 322:     b = xcalloc (n_bufs, sizeof *b); /** this statement fails **/
>> 398: }
>>
>> We also examined n_elide and n_bufs before that statement. Actually, they
>> are very
>> large:
>>
>> n_elide: 1125899906842624
>> n_bufs: 137438953474
>>
>> According to the following comment in the source file:
>>
>>> CAUTION: do not fail (out of memory) when asked to elide
>>> a ridiculous amount, but when given only a small input.  */
>>
>> We think this is a bug and bring this issue to your attention. Thanks!
> 
> There is the argument that we _should_ allocate
> everything up front to indicate immediately
> that the system can't (currently) support the requested operation,
> but given the 'currently' caveat above I guess it's
> more general to fail when we actually run out of mem?
> 
> So to do that we can either realloc our pointer array
> in chunks or use the simpler approach in the patch below,
> and take advantage of kernel overcommit strategies.
> (calloc is problematic for overcommit as zeroing the
> memory fully allocates it to the process).
> The caveat with that though is that it's dependent
> on the overcommit config for the kernel and possibly
> current memory conditions.
> 
> thanks,
> Pádraig.
> 
> diff --git a/src/head.c b/src/head.c
> index cf84a95..909be04 100644
> --- a/src/head.c
> +++ b/src/head.c
> @@ -197,7 +197,7 @@ copy_fd (int src_fd, FILE *o_stream, uintmax_t n_bytes)
>    return COPY_FD_OK;
>  }
> 
> -/* Print all but the last N_ELIDE lines from the input available via
> +/* Print all but the last N_ELIDE bytes from the input available via
>     the non-seekable file descriptor FD.  Return true upon success.
>     Give a diagnostic and return false upon error.  */
>  static bool
> @@ -314,18 +314,22 @@ elide_tail_bytes_pipe (const char *filename, int fd, uintmax_t n_elide_0)
>        size_t n_read;
>        bool buffered_enough;
>        size_t i, i_next;
> +      size_t n_alloc = 0;
>        char **b;
>        /* Round n_elide up to a multiple of READ_BUFSIZE.  */
>        size_t rem = READ_BUFSIZE - (n_elide % READ_BUFSIZE);
>        size_t n_elide_round = n_elide + rem;
>        size_t n_bufs = n_elide_round / READ_BUFSIZE + 1;
> -      b = xcalloc (n_bufs, sizeof *b);
> +      b = xnmalloc (n_bufs, sizeof *b);
> 
>        buffered_enough = false;
>        for (i = 0, i_next = 1; !eof; i = i_next, i_next = (i_next + 1) % n_bufs)
>          {
> -          if (b[i] == NULL)
> -            b[i] = xmalloc (READ_BUFSIZE);
> +          if (! buffered_enough)
> +            {
> +              b[i] = xmalloc (READ_BUFSIZE);
> +              n_alloc = i + 1;
> +            }
>            n_read = full_read (fd, b[i], READ_BUFSIZE);
>            if (n_read < READ_BUFSIZE)
>              {
> @@ -389,7 +393,7 @@ elide_tail_bytes_pipe (const char *filename, int fd, uintmax_t n_elide_0)
>          }
> 
>      free_mem:
> -      for (i = 0; i < n_bufs; i++)
> +      for (i = 0; i < n_alloc; i++)
>          free (b[i]);
>        free (b);

I expect to push soon, the attached more complete fix to realloc the array.

thanks,
Pádraig.

[head-elide-tail-pipe-mem.patch (text/x-patch, attachment)]

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