elf.c:3701: off += (m->sections[0]->vma - off) % bed->maxpagesize

Ian Lance Taylor ian@wasabisystems.com
Wed Jan 21 03:45:00 GMT 2004


Andrew Cagney <cagney@gnu.org> writes:

>  From elf.c:
> 
> 3701                off += (m->sections[0]->vma - off) % bed->maxpagesize;
> (gdb) print off
> $51 = 0x54
> (gdb) print m->sections[0]->vma
> $52 = 0x0
> (gdb) print bed->maxpagesize
> $53 = 0x10000
> 
> ok:
> 
> (gdb) ptype m->sections[0]->vma
> type = long unsigned int
> (gdb) ptype off
> type = long int
> (gdb) ptype bed->maxpagesize
> type = long unsigned int
> (gdb) print  off + (m->sections[0]->vma - off) % bed->maxpagesize
> $50 = 0x10000
> 
> broken:
> 
> (gdb) ptype m->sections[0]->vma
> type = long unsigned int
> (gdb) ptype off
> type = long long int
> (gdb) ptype bed->maxpagesize
> type = long unsigned int
> (gdb) print  off + (m->sections[0]->vma - off) % bed->maxpagesize
> $39 = 0x0
> 
> I can make it work with:
> 
> (gdb) print  off + (m->sections[0]->vma - off) % (ufile_ptr)
> bed->maxpagesize
> $40 = 0x10000
> 
> but is there a better way to write this expression? (that arithmetic
> strikes me as weird) and just how many others like this are lurking
> :-/ :-)?

We shouldn't be using % with a negative signed number.  That's just
not good.  We have to make sure either that the number is not
negative, or that the number has an unsigned type.  When the types are
all the same size they wind up as unsigned.  When file_ptr is a larger
signed type, the arithmetic winds up as signed long long, which is
wrong.

What we're trying to say here is something like ``increase OFF by the
least amount that will cause it to be equal to the VMA modulo the page
size.''  In other words, something like
    vma_offset = m->sections[0]->vma % bed->maxpagesize;
    off_offset = off % bed->maxpagesize;
    off += vma_offset - off_offset;
    if (vma_offset < off_offset)
      off += bed->maxpagesize;
which can be collapsed to the above expression when using unsigned
arithmetic.

Probably easiest to just cast OFF to ufile_ptr.

I doubt there is much of this sort of arithmetic--though of course it
appears again 15 lines lower in the function.

Ian



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