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Files open O_WRONLY not supported in fallocate emulation.

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In the posix_fallocate description in the manual we list various
drawbacks with the emulation, including the fact that a file opened
with O_APPEND fails with EBADF. Similarly a file opened with O_WRONLY
fails with EBADF. We must be able to emulate a compare-and-swap via
pread/compare/pwrite in order to make the emulation as safe as possible.
It is not acceptable to ignore the read failure because it could result
in significant data loss across all of the blocks. There is no other way
to make this work without a true atomic CAS and SIGBUS handler (which
is looking more attractive as a way to remove the race condition).

This patch adds O_WRONLY to the manual as another bullet to clarify the
limits of the emulation.

Manual looks good in PDF.
This commit is contained in:
Carlos O'Donell 2015-09-29 11:32:36 -04:00
parent a5721ebc68
commit 032f225008
2 changed files with 9 additions and 0 deletions
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5
ChangeLog
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@ -1,3 +1,8 @@
2015-09-29 Carlos O'Donell <carlos@redhat.com>
* manual/filesys.texi (Storage Allocation): Document that
posix_fallocate emulation fails when fd is open with O_WRONLY.
2015-09-28 Joseph Myers <joseph@codesourcery.com>
[BZ #19016]

4
manual/filesys.texi
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@ -3299,6 +3299,10 @@ There is a race condition if another thread or process modifies the
underlying file in the to-be-allocated area. Non-null bytes could be
overwritten with null bytes.
@item
If @var{fd} has been opened with the @code{O_WRONLY} flag, the function
will fail with an @code{errno} value of @code{EBADF}.
@item
If @var{fd} has been opened with the @code{O_APPEND} flag, the function
will fail with an @code{errno} value of @code{EBADF}.