GNU bug report logs - #18621
[BUG] wc -c incorrectly counts bytes in /sys

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From: Pádraig Brady <P <at> draigBrady.com>
To: Paul Eggert <eggert <at> cs.ucla.edu>
Cc: 18621 <at> debbugs.gnu.org, George Shuklin <george.shuklin <at> gmail.com>, Jim Meyering <jim <at> meyering.net>
Subject: bug#18621: [BUG] wc -c incorrectly counts bytes in /sys
Date: Fri, 03 Oct 2014 20:17:15 +0100

On 10/03/2014 07:47 PM, Paul Eggert wrote:
> On 10/03/2014 11:26 AM, Jim Meyering wrote:
>> That looks like a fine fix.
> 
> Unfortunately that fix would make 'wc -c' waaaaay slower for a file that consists entirely of a big hole.

True, which you could avoid by deferring to read() for empty files:

diff --git a/src/wc.c b/src/wc.c
index 1ff007d..f8176cc 100644
--- a/src/wc.c
+++ b/src/wc.c
@@ -235,6 +235,7 @@ wc (int fd, char const *file_x, struct fstatus *fstatus)
         fstatus->failed = fstat (fd, &fstatus->st);

       if (! fstatus->failed && S_ISREG (fstatus->st.st_mode)
+          && fstatus->st.st_blocks && fstatus->st.st_size
           && (current_pos = lseek (fd, 0, SEEK_CUR)) != -1
           && (end_pos = lseek (fd, 0, SEEK_END)) != -1)
         {

> How about if we change usable_st_size to return false for these proc files, with a heuristic as tight as we can make it, and to have coreutils check usable_st_size in more places.  Something like this, perhaps:
> 
>   /* Return a boolean indicating whether SB->st_size is correct. */
>   static inline bool
>   usable_st_size (struct stat const *sb)
>   {
>     if (S_ISREG (sb->st_mode))
>       {
>         /* proc files like /sys/kernel/vmcoreinfo are weird: their
>            st_size values do not reflect what's actually in them.
>            The following heuristic attempts to catch proc files without
>            catching many regular files that just happen to have the same
>            signature.  */
>         return ! (sb->st_uid == 0 && sb->st_gid == 0 && sb->st_blocks == 0
>                   && sb->st_size == ST_BLKSIZE (*sb));
>       }
>     return (S_ISLNK (sb->st_mode) || S_TYPEISSHM (sb) || S_TYPEISTMO (sb));
>   }
>
> and then review every place where coreutils currently uses st_size and prepend a check for usable_st_size if needed.

That would be usefult, and that check matches this case, however many are not
distinguishable. Consider: `stat /proc/$$/io` which has st_size and st_blocks = 0
Note my adjusted patch above handles both cases.

thanks,
Pádraig

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