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From: "qiyao at gcc dot gnu.org" <sourceware-bugzilla@sourceware.org>
To: gdb-prs@sourceware.org
Subject: [Bug gdb/17384] android arm gdb "Cannot access memory at address"
 when I "stepi" over "blx"
Date: Mon, 22 Sep 2014 13:12:00 -0000
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https://sourceware.org/bugzilla/show_bug.cgi?id=17384

Yao Qi <qiyao at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |qiyao at gcc dot gnu.org

--- Comment #22 from Yao Qi <qiyao at gcc dot gnu.org> ---
(In reply to Pedro Alves from comment #20)
> > (gdb) disassemble /r 0x4e0dcc20,+10
> > Dump of assembler code from 0x4e0dcc20 to 0x4e0dcc2a:
> >    0x4e0dcc20:  04 c0 9f e5 ldr r12, [pc, #4]   ; 0x4e0dcc2c
> >    0x4e0dcc24:  0c c0 8f e0 add r12, pc, r12
> >    0x4e0dcc28:  1c ff 2f e1 bx  r12
> > End of assembler dump.
> 
> I wonder whether that's some sort of trampoline the compiler/linker is
> generating
> and gdb is not grokking.
> 

Yes, it is a trampoline and GDB isn't aware of that.

See bfd/elf32-arm.c:

static const insn_sequence elf32_arm_stub_long_branch_any_thumb_pic[] =
{
  ARM_INSN (0xe59fc004),             /* ldr   ip, [pc, #4] */
  ARM_INSN (0xe08fc00c),             /* add   ip, pc, ip */
  ARM_INSN (0xe12fff1c),             /* bx    ip */
  DATA_WORD (0, R_ARM_REL32, 0),     /* dcd   R_ARM_REL32(X) */
};

We need to teach GDB to understand it in both arm_stub_unwind_sniffer and
arm_skip_stub.  I'll take a look.

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