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From: "vries at gcc dot gnu.org" <sourceware-bugzilla@sourceware.org>
To: gdb-prs@sourceware.org
Subject: [Bug tdep/29043] New: [gdb, i386, pie] Prologue skipper does not
 skip get_pc_thunk
Date: Mon, 11 Apr 2022 05:38:01 +0000
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https://sourceware.org/bugzilla/show_bug.cgi?id=3D29043

            Bug ID: 29043
           Summary: [gdb, i386, pie] Prologue skipper does not skip
                    get_pc_thunk
           Product: gdb
           Version: HEAD
            Status: NEW
          Severity: normal
          Priority: P2
         Component: tdep
          Assignee: unassigned at sourceware dot org
          Reporter: vries at gcc dot gnu.org
  Target Milestone: ---

Consider function bar in test-case gdb/testsuite/gdb.dwarf2/dw2-lines.c:
...
void
bar (void)
{
  asm ("bar_label: .globl bar_label");
  foo (1);
  asm ("bar_label_2: .globl bar_label_2");
  foo (2);
  asm ("bar_label_3: .globl bar_label_3");
  foo (3);
  asm ("bar_label_4: .globl bar_label_4");
  foo (4);
  asm ("bar_label_5: .globl bar_label_5");
}
...

When compiled for m32/pie:
...
$ gcc src/gdb/testsuite/gdb.dwarf2/dw2-lines.c -g -m32 -fPIE -pie
...
and setting a breakpoint at bar:
...
=E2=94=82    0x5655552d <bar>        push   %ebp=20=20=20=20=20=20=20=20=20=
=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=
=20=20=20=20=20
             =E2=94=82
=E2=94=82    0x5655552e <bar+1>      mov    %esp,%ebp=20=20=20=20=20=20=20=
=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=
=20=20
             =E2=94=82
=E2=94=82    0x56555530 <bar+3>      call   0x5655557e <__x86.get_pc_thunk.=
ax>=20=20=20=20=20=20=20=20=20
             =E2=94=82
=E2=94=82    0x56555535 <bar+8>      add    $0x1acb,%eax=20=20=20=20=20=20=
=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20
             =E2=94=82
=E2=94=82B+> 0x5655553a <bar+13>     push   $0x1=20=20=20=20=20=20=20=20=20=
=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=
=20=20=20=20=20
             =E2=94=82
=E2=94=82    0x5655553c <bar+15>     call   0x5655551d <foo>=20=20=20=20=20=
=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20
             ...
we see that it ends up after the get_pc_thunk, so apparently, with regular
debug info available, this is considered part of the prologue.

But when running the test-case, so with synthetic debug info, we run into a
FAIL:
...
Breakpoint 2, 0x56555540 in bar ()^M
(gdb) PASS: gdb.dwarf2/dw2-lines.exp: cv=3D2: cdw=3D32: lv=3D2: ldw=3D32: c=
ontinue to
breakpoint: foo \(1\)
next^M
Single stepping until exit from function bar,^M
which has no line number information.^M
0x56555587 in main ()^M
(gdb) FAIL: gdb.dwarf2/dw2-lines.exp: cv=3D2: cdw=3D32: lv=3D2: ldw=3D32: n=
ext to foo
(2)
...
because the breakpoint ends up before the call to get_pc_thunk:
...
=E2=94=82    0x5655553d <bar>        push   %ebp=20=20=20=20=20=20=20=20=20=
=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=
=20=20=20=20=20
             =E2=94=82
=E2=94=82    0x5655553e <bar+1>      mov    %esp,%ebp=20=20=20=20=20=20=20=
=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=
=20=20
             =E2=94=82
=E2=94=82B+> 0x56555540 <bar+3>      call   0x5655558e <__x86.get_pc_thunk.=
ax>=20=20=20=20=20=20=20=20=20
             =E2=94=82
=E2=94=82    0x56555545 <bar+8>      add    $0x1abb,%eax=20=20=20=20=20=20=
=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20
             =E2=94=82
=E2=94=82    0x5655554a <bar+13>     push   $0x1=20=20=20=20=20=20=20=20=20=
=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=
=20=20=20=20=20
             =E2=94=82
=E2=94=82    0x5655554c <bar+15>     call   0x5655552d <foo>=20=20=20=20=20=
=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20
             ...
which is in a part that the synthetic debug info doesn't provide line number
information for, hence the FAIL.

AFAIU, the synthetic debug info is incomplete and therefore gdb needs to re=
ly
on the port's prologue skipper, which apparently doesn't handle get_pc_thun=
k.

The test-case itself is not meant to test this behaviour, and can be fixed =
by
setting a breakpoint on bar_label instead of bar.

--=20
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