[RFC] PR 20939: Handle error in disassembly

Yao Qi
2016-12-12T10:48:58Z

Hi,
GDB calls some APIs from opcodes to do disassembly and provide some
call backs. This model makes troubles on C++ exception unwinding,
because GDB is a C++ program, and opcodes is still compiled as C.
As we can see, frame #10 and #12 are C++, while #frame 11 is C,

#10 0x0000000000544228 in memory_error (err=TARGET_XFER_E_IO, memaddr=<optimized out>) at ../../binutils-gdb/gdb/corefile.c:237
#11 0x00000000006b0a54 in print_insn_aarch64 (pc=0, info=0xffffffffeeb0) at ../../binutils-gdb/opcodes/aarch64-dis.c:3185
#12 0x0000000000553590 in gdb_pretty_print_insn (gdbarch=gdbarch@entry=0xbbceb0, uiout=uiout@entry=0xbc73d0, di=di@entry=0xffffffffeeb0,
insn=0xffffffffed40, insn@entry=0xffffffffed90, flags=flags@entry=0,

C++ exception unwinder can't go across frame #11 unless it has
unwind table. However, C program on many architectures doesn't
have it in default. As a result, GDB aborts, which is described
in PR 20939.

This is not the first time we see this kind of problem. We've
had a commit 89525768cd086a0798a504c81fdf7ebcd4c904e1
"Propagate GDB/C++ exceptions across readline using sj/lj-based TRY/CATCH".
We can fix the disassembly bug in a similar way, this is the option one.

Alternatively, we can do more changes in opcodes, because opcodes is
built together with gdb. Don't throw exception in dis_asm_memory_error,
and only throw exception if the return value of print_insn_$ARCH is -1
in GDB. This is the option two, which is demonstrated by the patch
below. This requires every print_insn_$ARCH function return -1 on
memory error, but msp430 and m68k don't follow this convention yet.

Which option do you prefer? If we prefer option one, the change is
only within the GDB scope. If we prefer option two, it goes out to
opcodes, and I'll bring the discussion to binutils. I prefer this
one.

Note that, no matter which option do we take, the fix should be
backported to 7.12 branch, in which GDB can still be built as a C
program.

--
Yao (é½å°§)

diff --git a/gdb/aarch64-tdep.c b/gdb/aarch64-tdep.c
index 0175630..4f5f056 100644
--- a/gdb/aarch64-tdep.c
+++ b/gdb/aarch64-tdep.c
@@ -1771,8 +1771,15 @@ aarch64_dwarf_reg_to_regnum (struct gdbarch *gdbarch, int reg)
static int
aarch64_gdb_print_insn (bfd_vma memaddr, disassemble_info *info)
{
+ int ret;
+
info->symbols = NULL;
- return print_insn_aarch64 (memaddr, info);
+ ret = print_insn_aarch64 (memaddr, info);
+
+ if (ret == -1)
+ memory_error (TARGET_XFER_E_IO, memaddr);
+
+ return ret;
}

/* AArch64 BRK software debug mode instruction.
diff --git a/gdb/disasm.c b/gdb/disasm.c
index 6f9f5f9..437b64c 100644
--- a/gdb/disasm.c
+++ b/gdb/disasm.c
@@ -137,7 +137,7 @@ static void
dis_asm_memory_error (int err, bfd_vma memaddr,
struct disassemble_info *info)
{
- memory_error (TARGET_XFER_E_IO, memaddr);
+ /*memory_error (TARGET_XFER_E_IO, memaddr);*/
}

/* Like print_address with slightly different parameters. */