linux-kernel - Re: Proposal to fix pwrite with O

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Message-ID: <87d0b942lp.fsf@oldenburg2.str.redhat.com>
Date:   Fri, 24 Jan 2020 10:37:22 +0100
From:   Florian Weimer <fweimer@...hat.com>
To:     Rich Felker <dalias@...c.org>
Cc:     linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-api@...r.kernel.org, Alexander Viro <viro@...iv.linux.org.uk>
Subject: Re: Proposal to fix pwrite with O_APPEND via pwritev2 flag

* Rich Felker:

> There's a longstanding unfixable (due to API stability) bug in the
> pwrite syscall:
>
> http://man7.org/linux/man-pages/man2/pwrite.2.html#BUGS
>
> whereby it wrongly honors O_APPEND if set, ignoring the caller-passed
> offset. Now that there's a pwritev2 syscall that takes a flags
> argument, it's possible to fix this without breaking stability by
> adding a new RWF_NOAPPEND flag, which callers that want the fixed
> behavior can then pass.
>
> I have a completely untested patch to add such a flag, but would like
> to get a feel for whether the concept is acceptable before putting
> time into testing it. If so, I'll submit this as a proper patch with
> detailed commit message etc. Draft is below.

Has this come up before?

I had already written a test case and it turns out that an O_APPEND
descriptor does not protect the previously written data in the file:

openat(AT_FDCWD, "/tmp/append-truncateuoRexJ", O_RDWR|O_CREAT|O_EXCL, 0600) = 3
write(3, "@", 1)                        = 1
close(3)                                = 0
openat(AT_FDCWD, "/tmp/append-truncateuoRexJ", O_WRONLY|O_APPEND) = 3
ftruncate(3, 0)                         = 0

So at least it looks like there is no security issue in adding a
RWF_NOAPPEND flag.

Thanks,
Florian