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Re: [patch] Fix 64-bit->32-bit vDSO reporting

The problem is indeed obvious.  I really wish there were a better way to
detect the bit-width of the process than looking at /proc/PID/exe, but
perhaps there really isn't.

What about just looking at the first 8 bytes of the auxv data?  The first
element is never AT_NULL, so it will have a nonzero a_type.  No a_type
value will ever be more than UINT32_MAX.  So for big-endian, the first four
bytes being zero says it's 64-bit.  For little-endian, it's not so easy
without assuming that the first a_val is nonzero.  That's actually reliable
enough in practice, but it feels wrong.  Maybe there is something better
for little-endian that I'm not thinking of.

> +  ssize_t nread = read (fd, &buf, sizeof buf);

Use pread_retry.

> +  unsigned char buf[5];
> +  close (fd);
> +  if (nread != sizeof (buf) || buf[0] != ELFMAG0 || buf[1] != ELFMAG1
> +      || buf[2] != ELFMAG2 || buf[3] != ELFMAG3
> +      || (buf[4] != ELFCLASS64 && buf[4] != ELFCLASS32))
> +    return -1;
> +
> +  return buf[4] == ELFCLASS64 ? 64 : 32;

Please use the EI_* macros instead of literal indices and size.

I'd just make it return the [EI_CLASS] value and compare against
that in grovel_auxv.


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