bug#23825: maint: avoid md5sum.c warning from bleeding-edge gcc's -Wstrict-overflow

[Paul Eggert]

On 06/22/2016 04:37 PM, Jim Meyering wrote:
> Building with a recent gcc-7 failed, so I wrote the attached patch. I
> think we'll never have 2^31 command line arguments
I would rather fix the code so that it works even if argc is initially 
INT_MAX; the code currently has undefined behavior in that case. I have 
drafted a patch for this (attached; does not have changelog entries yet) 
and will test it a bit. I assume this will fix the warning (don't have 
gcc-7 offhand so can't test this).

As the patch shows, a couple of other coreutils programs have a similar 
bug; I'm a bit surprised gcc-7 didn't complain about them.

Incidentally, 'yes' has a different bug: it mishandles the case where 
'write' succeeds but returns a value less than the buffer size. I'll try 
to look into that too. Simplest would be to use stdio (the comments 
indicate this has performance issues but I don't know what they are, 
anyway correctness trumps performance).

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