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Date: Fri, 14 Feb 2025 13:49:52 +0100
To: cygwin AT cygwin DOT com
Subject: Re: SEEK_DATA should fail at EOF (was: coreutils-9.6-1 (TEST): cp:
 infinite SEEK_SET/DATA/HOLE loop if file is compressed)
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From: Corinna Vinschen via Cygwin <cygwin AT cygwin DOT com>
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On Feb 14 12:24, Christian Franke via Cygwin wrote:
> Christian Franke via Cygwin wrote:
> > Testcase:
> > [...]
> > Upstream bug?
> 
> Possibly not. A closer look shows that the main loop in copy.c:lseek_copy()
> expects that SEEK_DATA fails with ENXIO at EOF.
> 
> https://github.com/coreutils/coreutils/blob/v9.6/src/copy.c#L543
> 
>  lseek_copy(..., off_t ext_start, ...)
>  {
>    ...
>    while (0 <= ext_start) {
>      {
>       ...
>       ext_start = lseek (src_fd, dest_pos, SEEK_DATA);
>       if (ext_start < 0 && errno != ENXIO)
>         goto cannot_lseek;
>      }
>    ...
> }
> 
> This works on Linux (checked on Debian 12) but Cygwin returns the offset if
> it is equal to the file size.
> 
> Recent POSIX says:
> "[ENXIO] The whence argument is SEEK_HOLE or SEEK_DATA, and offset is
> greater than or equal to the file size"
> https://pubs.opengroup.org/onlinepubs/9799919799/functions/lseek.html
> 
> But (at least older) Linux man pages suggest that Cygwin behavior may be
> correct also:
> "In the simplest implementation, a filesystem can support the operations by
> making ... SEEK_DATA always return offset."
> "ENXIO - whence is SEEK_DATA or SEEK_HOLE, and offset is beyond the end of
> the file"
> https://man7.org/linux/man-pages/man2/lseek.2.html

In terms of "older", this text is still part of
https://man7.org/linux/man-pages/man2/lseek.2.html.

But we do return ENXIO.  However...

> Hmm... does "beyond" mean '>=' or '>' ?

...do you think this fixes it?

diff --git a/winsup/cygwin/fhandler/base.cc b/winsup/cygwin/fhandler/base.cc
index 8f3dbd4ed51a..79dfaaa5987a 100644
--- a/winsup/cygwin/fhandler/base.cc
+++ b/winsup/cygwin/fhandler/base.cc
@@ -1144,7 +1144,7 @@ fhandler_base::lseek (off_t offset, int whence)
 	    return -1;
 	  }
 	/* Per Linux man page, ENXIO if offset is beyond EOF */
-	if (offset > fsi.EndOfFile.QuadPart)
+	if (offset >= fsi.EndOfFile.QuadPart)
 	  {
 	    set_errno (ENXIO);
 	    return -1;


Corinna

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