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Re: [PATCH v3] 9pfs: Fix segfault in do_readdir_many caused by struct di


From: Philippe Mathieu-Daudé
Subject: Re: [PATCH v3] 9pfs: Fix segfault in do_readdir_many caused by struct dirent overread
Date: Fri, 4 Feb 2022 20:31:41 +0100
User-agent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10.15; rv:91.0) Gecko/20100101 Thunderbird/91.5.1

On 4/2/22 17:04, Christian Schoenebeck wrote:
On Freitag, 4. Februar 2022 16:54:12 CET Philippe Mathieu-Daudé wrote:
On 4/2/22 16:50, Dmitry V. Levin wrote:
On Fri, Feb 04, 2022 at 06:32:07PM +0300, Vitaly Chikunov wrote:
[...]

struct dirent *
qemu_dirent_dup(struct dirent *dent)
{

      size_t sz = offsetof(struct dirent, d_name) + _D_EXACT_NAMLEN(dent)
      + 1;

But d_namlen is not populated by synth_direntry, so this will lead to
a bug too. Idea is that qemu_dirent_dup handles real dirents and
simulated (underpopulated) dirents.

Also Linux does not have d_namlen AFAIK, thus this code will not provide
any speed up in most cases (and always fallback to strlen), unlike if we
use d_reclen.

Also, I m not sure if _D_EXACT_NAMLEN is defined on all systems, so this
needs ifdefs too.

Yes, _D_EXACT_NAMLEN() is a GNU extension, it was introduced in glibc
back in 1996 but some popular libcs available for Linux do not provide
this macro.

Can't we define _D_EXACT_NAMLEN() if not available?

It is not that trivial.

With recent macOS patch set in mind: macOS does not have any of these macros
either. It does have d_namlen and d_reclen though. Keep in mind though that
macOS also has d_seekoff which is almost always zero though.

So please, don't blindly define something, test it! On doubt I stick with
Vitaly's solution, because it just works^TM.

Note I haven't NAck'ed this approach, I am simply looking at a better
alternative if possible.



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