[LLVMdev] [lld] TBSS wrong size

[Shankar Easwaran]

It looks like TBSS size is wrong.The bug is in the function
Segment<ELFT>::assignVirtualAddress as you pointed.

On Wed, Jun 3, 2015 at 9:13 AM, Shankar Easwaram <shankarke at gmail.com>
wrote:

> Ah this is a nasty bug. You might want to checkout if some of the TLS
> variables are allocated elsewhere? It cannot just disappear from the output.
>
>
> > On Jun 3, 2015, at 08:16, Adhemerval Zanella <
> adhemerval.zanella at linaro.org> wrote:
> >
> > Hi,
> >
> > Yes, ldd is generating wrong tbss size.  It is just considering one tbss
> section
> > and not calculating all sections from all objects.  The following
> example on
> > x86_64 shows the issue:
> >
> > --- t0.c ---
> >
> > #include <stdio.h>
> >
> > extern __thread int t0;
> > extern __thread int t1;
> > extern __thread int t2;
> > extern __thread int t3;
> >
> > __thread int t4;
> > __thread int t5;
> > __thread int t6;
> > __thread int t7;
> >
> > int main ()
> > {
> >  t4 = 1;
> >  t5 = 2;
> >  t6 = 3;
> >  t7 = 4;
> >
> >  printf ("%i %i %i %i\n", t0, t1, t2, t3);
> >  printf ("%i %i %i %i\n", t4, t5, t6, t7);
> >
> >  return 0;
> > }
> >
> > --- t1.c ---
> >
> > __thread int t0;
> > __thread int t1;
> > __thread int t2;
> > __thread int t3;
> >
> > -------------
> >
> > If you build with lld you will see:
> >
> > $ ./t-lld
> > 1 1 1 1
> > 1 2 3 4
> >
> > Because t{4,5,6,7} space is not taking in consideration.  In fact if you
> > check the resulting tbss for the test:
> >
> >  [15] .tbss             NOBITS           0000000000401000  00001000
> >       0000000000000010  0000000000000000 WAT       0     0     4
> >
> > Its size is just 0x10 (4 int), where it should be 0x20 (8 ints).
> >
> >
> >> On 02-06-2015 21:14, Shankar Easwaram wrote:
> >> Are you saying it generates wrong section size?  Tbss is very special
> and I checked the behavior on X86_64 to model it. It does not account for
> virtual address increase as libc allocates it.
> >>
> >>
> >>> On Jun 2, 2015, at 17:20, Adhemerval Zanella <
> adhemerval.zanella at linaro.org> wrote:
> >>>
> >>> Hi,
> >>>
> >>> I am tracking some TLS issues with lld and found that it is
> >>> generating wrong tbss size for case where multiple modules
> >>> have non initialized threads variables.  For instance:
> >>>
> >>> -- t0.c --
> >>>
> >>> __thread int x0;
> >>> __thread int x1;
> >>> __thread int x2;
> >>>
> >>> extern __thread int e0;
> >>> extern __thread int e1;
> >>> extern __thread int e2;
> >>> extern __thread int e3;
> >>>
> >>> int foo0 ()
> >>> {
> >>> return x0;
> >>> }
> >>>
> >>> int main ()
> >>> {
> >>> return x0;
> >>> }
> >>>
> >>> -- t1.c --
> >>>
> >>> __thread int e0;
> >>> __thread int e1;
> >>> __thread int e2;
> >>> __thread int e3;
> >>>
> >>> ---
> >>>
> >>> lld is generating (for aarch64):
> >>>
> >>> [14] .tbss             NOBITS           0000000000401000  00001000
> >>>      0000000000000010  0000000000000000 WAT       0     0     4
> >>>
> >>> Where is just taking in consideration the largest tbss segment, not all
> >>> from all objects.  ld generates a correct output:
> >>>
> >>> [17] .tbss             NOBITS           0000000000410dec  00000dec
> >>>      000000000000001c  0000000000000000 WAT       0     0     4
> >>>
> >>> My initial idea is that 'lib/ReaderWriter/ELF/SegmentChunks.cpp' /
> >>> Segment<ELFT>::assignVirtualAddress is setting wrong slice values,
> however
> >>> playing with this I could not find a correct logic to handle the TBSS.
> >>>
> >>> Any ideas where lld is possible messing the TBSS segments sizes?
>
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