Re: [Qemu-devel] [PATCH for-2.6 1/3] QemuOpts: Fix qemu_opts_foreach() dangling location regression

[Eric Blake <eblake@redhat.com>]

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On 04/27/2016 08:29 AM, Markus Armbruster wrote:
> qemu_opts_foreach() pushes and pops a Location with automatic storage
> duration.  Except it fails to pop when @func() returns non-zero.
> cur_loc then points to unused stack space, and will most likely get
> clobbered in short order.
> 
> Clobbered cur_loc can make loc_pop() and error_print_loc() crash or
> report bogus locations.
> 
> Affects several qemu command line options as well as qemu-img,
> qemu-io, qemu-nbd -object, and blkdebug's configuration file.
> 
> Broken in commit a4c7367, v2.4.0.

Latent bug means it's not a regression between 2.5 and 2.6, but I agree
that if there is time to get this in 2.6, it is worth having.  It's a
shame that valgrind doesn't catch use of stale stack space.


> cur_loc then points to where qemu_opts_foreach()'s Location used to
> be, i.e. unused stack space.  With optimization, this Location doesn't
> get clobbered for me, and also happens to be the correct location.
> Without optimization, it does get clobbered in a way that makes
> error_report_err() report no location.

And that explains why some people were having problems reproducing.

> 
> Signed-off-by: Markus Armbruster <armbru@redhat.com>
> ---
>  util/qemu-option.c | 6 +++---
>  1 file changed, 3 insertions(+), 3 deletions(-)

Reviewed-by: Eric Blake <eblake@redhat.com>

-- 
Eric Blake   eblake redhat com    +1-919-301-3266
Libvirt virtualization library http://libvirt.org


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