strace seg with select and -ve nfds

[Dr. David Alan Gilbert]

* Denys Vlasenko (dvlasenk at redhat.com) wrote:
> On 11/05/2013 07:27 PM, Dr. David Alan Gilbert wrote:
> > * Denys Vlasenko (dvlasenk at redhat.com) wrote:
> >> On the related note, how are we doing in "stracing 32-bit app
> >> with 64-bit strace on a big-endian machine" case?
> >> In that case, sizeof(long) is important...
> >> I dread to think about that:(
> > 
> > Yeh that's kind of related to the question I asked in my 1st post;
> > since we're getting passed args here as long* and this is actually
> > an int does this work at all on BE machines?
> > Is there GET_ARG_INT thing in strace this routine should be using?
> 
> You misunderstood me. The question is not about arg being longs
> or ints - that is properly covered (or else EVERY syscall would be
> horribly broken).
> 
> I'm talking about the long[] arrays pointed by arg[1,2,3].
> In 32-bit process, those arrays have 32-bit "longs".
> If strace is 64-bit, strace's "longs" are 64-bit.
> 
> Finding an Nth bit via
> 
> 	long_array[fd / BITS_PER_LONG] & (1UL << (fd % BITS_PER_LONG))
> 
> works if we have a correct idea about sizeof(long),
> and it also works correctly on little endian
> even if we don't:
> 
> 	fd = 32
> 	if long is 32-bit:
> 		long_array[1] & (1 << 0) -- checks bit 0 in 4th byte
> 	if long is 64-bit:
> 		long_array[0] & (1 << 32) -- checks bit 0 in 4th byte
> 
> But on big-endian we have a disaster
> 
> 	fd = 32
> 	if long is 32-bit:
> 		long_array[1] & (1 << 0) -- checks bit 0 in 7th byte
> 	if long is 64-bit:
> 		long_array[0] & (1 << 32) -- checks bit 0 in 3rd byte
> 
> NOT THE SAME!

Yeh ok, but now we're indexing into the array our selves we could
just treat it as a series of bytes and pass in flags for endianness/long
size and work it out the hard way.

Dave
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