On 2013.12.02 at 15:15 +0100, Antoine Pelisse wrote:
> On Mon, Dec 2, 2013 at 1:57 PM, Markus Trippelsdorf
> <markus@xxxxxxxxxxxxxxx> wrote:
> > When git is compiled with current gcc and "-march=native"
> > git-blame segfaults:
> >
> > For example:
> >
> > % gdb --args /var/tmp/git/git-blame gcc/tree-object-size.c
> > ...
> > Program received signal SIGSEGV, Segmentation fault.
> > 0x0000000000000000 in ?? ()
> > (gdb) bt
> > #0 0x0000000000000000 in ?? ()
> > #1 0x000000000051240d in xdl_emit_hunk_hdr (s1=s1@entry=30, c1=<optimized out>, s2=s2@entry=30, c2=c2@entry=6, func=func@entry=0x7fffffffd2d8 "", funclen=0,
> > ecb=ecb@entry=0x7fffffffd580) at xdiff/xutils.c:460
> > #2 0x0000000000512af7 in xdl_emit_diff (xe=0x7fffffffd390, xscr=<optimized out>, ecb=0x7fffffffd580, xecfg=0x7fffffffd590) at xdiff/xemit.c:237
>
> xdl_emit_diff() should not be called, because xecfg->hunk_func should
> be non-null and called instead. xld_emit_diff() is the one that needs
> outf to be set.
>
> > #3 0x0000000000510a5d in xdl_diff (mf1=mf1@entry=0x7fffffffd510, mf2=mf2@entry=0x7fffffffd520, xpp=xpp@entry=0x7fffffffd570, xecfg=xecfg@entry=0x7fffffffd590,
> > ecb=ecb@entry=0x7fffffffd580) at xdiff/xdiffi.c:601
>
> Here we decide that xecfg->hunk_func is empty, and ef is set to xdl_emit_diff.
>
> > #4 0x000000000050b005 in xdi_diff (mf1=<optimized out>, mf2=<optimized out>, xpp=xpp@entry=0x7fffffffd570, xecfg=xecfg@entry=0x7fffffffd590, xecb=xecb@entry=0x7fffffffd580)
> > at xdiff-interface.c:136
> > #5 0x00000000004104df in diff_hunks (file_a=<optimized out>, file_b=<optimized out>, ctxlen=ctxlen@entry=0, hunk_func=hunk_func@entry=0x411320 <blame_chunk_cb>,
> > cb_data=cb_data@entry=0x7fffffffd830) at builtin/blame.c:105
>
> As we can see in your code below, ecb.outf is not set here, because
> it's not needed by hunk_func.
> hunk_func is set to blame_chunck_cb, passed by the function below.
>
> > #6 0x0000000000412b54 in pass_blame_to_parent (parent=0x11da810, target=0x11dab50, sb=0x7fffffffd6e0) at builtin/blame.c:815
>
> diff_hunks() is called with blame_chunck_cb as hunk_func.
>
> I think the best thing to do is to find out where xecfg->hunk_func
> loses the blame_chunck_cb value to NULL. You could start with a
> watchpoint.
It happens in diff_hunks (from builtin/blame.c):
(gdb) up
#5 0x00000000004104df in diff_hunks (file_a=<optimized out>, file_b=<optimized out>, ctxlen=ctxlen@entry=0, hunk_func=hunk_func@entry=0x411320 <blame_chunk_cb>,
cb_data=cb_data@entry=0x7fffffffd830) at builtin/blame.c:105
105 return xdi_diff(file_a, file_b, &xpp, &xecfg, &ecb);
(gdb) l
100
101 xpp.flags = xdl_opts;
102 xecfg.ctxlen = ctxlen;
103 xecfg.hunk_func = hunk_func;
104 ecb.priv = cb_data;
105 return xdi_diff(file_a, file_b, &xpp, &xecfg, &ecb);
106 }
107
108 /*
109 * Prepare diff_filespec and convert it using diff textconv API
(gdb) p hunk_func
$1 = (xdl_emit_hunk_consume_func_t) 0x411320 <blame_chunk_cb>
(gdb) p xecfg.hunk_func
$2 = (xdl_emit_hunk_consume_func_t) 0x0
(gdb) p xecfg
$3 = {
ctxlen = 0,
interhunkctxlen = 0,
flags = 0,
find_func = 0x0,
find_func_priv = 0x0,
hunk_func = 0x0
}
>
> Would that mean that gcc is doing some steps in the wrong order ? That
> is setting xecfg.hunk_func and then emptying the structure ? I've
> already had a similar bug, but that's very unfortunate.
Yes. I think this might be the case:
(gdb) disass
Dump of assembler code for function diff_hunks:
0x0000000000410460 <+0>: sub $0x58,%rsp
0x0000000000410464 <+4>: xor %eax,%eax
0x0000000000410466 <+6>: mov %eax,%r9d
0x0000000000410469 <+9>: add $0x20,%eax
0x000000000041046c <+12>: cmp $0x20,%eax
0x000000000041046f <+15>: movq $0x0,0x20(%rsp,%r9,1)
0x0000000000410478 <+24>: movq $0x0,0x28(%rsp,%r9,1)
0x0000000000410481 <+33>: movq $0x0,0x30(%rsp,%r9,1)
0x000000000041048a <+42>: movq $0x0,0x38(%rsp,%r9,1)
0x0000000000410493 <+51>: jb 0x410466 <diff_hunks+6>
0x0000000000410495 <+53>: lea 0x20(%rsp),%r10
0x000000000041049a <+58>: mov %rdx,0x20(%rsp)
0x000000000041049f <+63>: mov %rcx,0x48(%rsp)
0x00000000004104a4 <+68>: add %r10,%rax
0x00000000004104a7 <+71>: mov %r8,0x10(%rsp)
0x00000000004104ac <+76>: mov %rsp,%rdx
0x00000000004104af <+79>: movq $0x0,(%rax)
0x00000000004104b6 <+86>: movq $0x0,0x8(%rax)
0x00000000004104be <+94>: lea 0x10(%rsp),%r8
0x00000000004104c3 <+99>: movslq 0x171882(%rip),%rax # 0x581d4c <xdl_opts>
0x00000000004104ca <+106>: mov %r10,%rcx
0x00000000004104cd <+109>: movq $0x0,0x18(%rsp)
0x00000000004104d6 <+118>: mov %rax,(%rsp)
0x00000000004104da <+122>: callq 0x50aee0 <xdi_diff>
=> 0x00000000004104df <+127>: add $0x58,%rsp
0x00000000004104e3 <+131>: retq
End of assembler dump.
--
Markus
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