[PATCH] v4l2: Fix compile of mpers-m32 API

[Dmitry V. Levin]

On Sun, Aug 30, 2020 at 02:07:34PM +0200, Hauke Mehrtens wrote:
> On 8/30/20 12:54 PM, Eugene Syromyatnikov wrote:
> > On Sun, Aug 30, 2020 at 12:10 PM Hauke Mehrtens <hauke at hauke-m.de> wrote:
> >>
> >> In the mpers-m32 API the value64 member of the struct_v4l2_ext_control
> >> structure is of type int32_t.
> > 
> > mpers headers are generated, and the fact that int64_t[1] is 32-bit
> > wide when -m32 compiler option is used is rather a bug on libc's part.
> > See also [2].
> > 
> > [1] https://gitlab.com/strace/strace/-/blob/v5.8/types/v4l2.h#L175
> > [2] https://github.com/strace/musl/commit/6a63dbca01b26dd68483429746ec90e34551ace4
> > 
> 
> Hi Eugene,
> 
> I did this build in OpenWrt with musl libc 1.1.24 + gcc 8.4.0 for x86_64.
> 
> The file mpers-mx32/struct_v4l2_ext_control.h looks like this:
> -----------------------------------------------------------
> ....
> int32_t value;
> int32_t value64;
> ....
> } ;
> } ATTRIBUTE_PACKED mx32_struct_v4l2_ext_control;
> #define MPERS_mx32_struct_v4l2_ext_control mx32_struct_v4l2_ext_control
> -----------------------------------------------------------
> The mpers-m32/struct_v4l2_ext_control.h file looks similar.
> 
> When I compile this on my Debian with glibc and gcc 8.3.0 the value64 is
> of type int64_t.

That's because gcc -m32 is not supported by your musl libc.
We have to patch musl [2] to workaround this musl bug.

> Where are these structures in mpers-m32 and mpers-mx32 generated? It
> looks like some script which generates them converts the int64_t into a
> int32_t under some conditions.

Yes, all these structures are generated at build time using gcc -m32 and
gcc -mx32, respectively.

> I see that Apline just deactivated mpers support, I think that is also
> ok for OpenWrt:
> https://git.alpinelinux.org/aports/commit/main/strace/APKBUILD?id=69d6896b7d57d5e7dc9596fd3b4721205cb7bca8

If your x86_64 kernel is configured with either CONFIG_IA32_EMULATION
or CONFIG_X86_X32 enabled, then you shouldn't deactivate mpers support,
otherwise you're building a strace that won't be able to handle compat
syscalls on x86_64 properly.


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