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Re: [Qemu-devel] [for-2.10 PATCH v2] 9pfs: local: fix fchmodat_nofollow(
From: |
Eric Blake |
Subject: |
Re: [Qemu-devel] [for-2.10 PATCH v2] 9pfs: local: fix fchmodat_nofollow() limitations |
Date: |
Wed, 9 Aug 2017 10:59:46 -0500 |
User-agent: |
Mozilla/5.0 (X11; Linux x86_64; rv:52.0) Gecko/20100101 Thunderbird/52.2.1 |
On 08/09/2017 10:22 AM, Greg Kurz wrote:
>>>
>>> The solution is to use O_PATH: openat() now succeeds in both cases, and we
>>> can ensure the path isn't a symlink with fstat(). The associated entry in
>>> "/proc/self/fd" can hence be safely passed to the regular chmod() syscall.
>>
>> Hey - should we point this out as a viable solution to the glibc folks,
>> since their current user-space emulation of AT_SYMLINK_NOFOLLOW is broken?
>>
>
> Probably. What's the best way to do that ?
I've added a comment to
https://sourceware.org/bugzilla/show_bug.cgi?id=14578; you'll also want
to point to the lkml discussion in that bug. And reading that bug, it
also looks like your hack with /proc/self/fd has been proposed by Rich
Felker since 2013! (although fstat() didn't work until Linux 3.6, even
though O_PATH predates that time) - so there is that one additional
concern of whether we need to cater to the window of kernels where
O_PATH exists but fstat() on that fd can't learn whether we opened a
symlink.
--
Eric Blake, Principal Software Engineer
Red Hat, Inc. +1-919-301-3266
Virtualization: qemu.org | libvirt.org
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