[strace/strace] static assert (#133)

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Thu Apr 16 21:51:28 UTC 2020

On Fri, Apr 10, 2020 at 01:20:03PM -0700, Jory A. Pratt wrote:
> In file included from static_assert.h:11,
>                  from v4l2.c:13:
> xlat/v4l2_ioctl_cmds.h:43:1: error: static assertion failed: "VIDIOC_QUERYBUF != _IOWR(\'V\',   9, struct_v4l2_buffer)"
>    43 | static_assert((VIDIOC_QUERYBUF) == (_IOWR('V',   9, struct_v4l2_buffer)), "VIDIOC_QUERYBUF != _IOWR('V',   9, struct_v4l2_buffer)");
>       | ^~~~~~~~~~~~~
> xlat/v4l2_ioctl_cmds.h:64:1: error: static assertion failed: "VIDIOC_QBUF != _IOWR(\'V\',  15, struct_v4l2_buffer)"
>    64 | static_assert((VIDIOC_QBUF) == (_IOWR('V',  15, struct_v4l2_buffer)), "VIDIOC_QBUF != _IOWR('V',  15, struct_v4l2_buffer)");
>       | ^~~~~~~~~~~~~
> xlat/v4l2_ioctl_cmds.h:71:1: error: static assertion failed: "VIDIOC_DQBUF != _IOWR(\'V\',  17, struct_v4l2_buffer)"
>    71 | static_assert((VIDIOC_DQBUF) == (_IOWR('V',  17, struct_v4l2_buffer)), "VIDIOC_DQBUF != _IOWR('V',  17, struct_v4l2_buffer)");
> This is build using linux-headers-5.6 with musl-1.2.0, this is a regression as 5.5 compiles fine.

The reason why 5.5 compiles fine is that the check was introduced in 5.6,
but the problem lies elsewhere.

<linux/videodev2.h> use struct v4l2_buffer which in turn uses struct timeval.

The size of struct timeval on a traditional 32-bit architecture
like x86 depends on libc and its version thanks to time_t problem.

As result, the value of VIDIOC_QUERYBUF on a traditional 32-bit
architecture like x86 depends on libc and its version.

When v4l2.c is compiled in mpers mode to handle x86 and x32 personality
syscalls on x86_64, the value for VIDIOC_QUERYBUF is taken from the
appropriate linux/*/ioctls_inc*.h file.  There is only one value there for
each architecture, not two.  If you are unhappy enough to have time_t size
different from the one used to generate that ioctls_inc file, you get this
failed assertion.

The kernel starting with Linux kernel commit v5.6-rc1~93^2~102
supports both sets of ioctl values, but strace will recognize
and decode only one of them so far.

I don't see an easy fix of this issue for early adopters of 64-bit
time_t on 32-bit architectures, sorry.


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