Re: [PATCH] testsuite/gdb.dwarf2: Fix for dw2-dos-drive failure on ARM

[Omair Javaid <omair.javaid@linaro.org>]

On 3 December 2013 02:16, Omair Javaid <omair.javaid@linaro.org> wrote:
>
> On 10/01/2013 08:34 PM, Pedro Alves wrote:
>> On 10/01/2013 09:32 AM, Omair Javaid wrote:
>>> On 19 September 2013 20:53, Pedro Alves <palves@redhat.com> wrote:
>>>> Please don't top post.
>>>>
>>>> On 09/19/2013 04:23 PM, Omair Javaid wrote:
>>>>> Thanks everyone for the feedback.
>>>>>
>>>>> I am getting following problem with 1byte text section in the dw2-dos-drive.exp
>>>>>
>>>>> (gdb) PASS: gdb.dwarf2/dw2-dos-drive.exp: set breakpoint pending off
>>>>> break 'z:file.c':func
>>>>> Cannot access memory at address 0x0
>>>>>
>>>>> When I change this to 4bytes the problem gets fixed. That is why I
>>>>> thought this could be an unaligned illegal memory access but I accept
>>>>> that the above comments verify that its not a alignment issue.
>>>>>
>>>>> Can anyone help me figure out what could be the cause of this problem?
>>>>
>>>> Breakpoint instructions on ARM are 4-byte wide.  It sounds like
>>>> GDB is trying to read the memory at the breakpoint's address, and
>>>> that fails (that error message comes from GDB, not the program).
>>>> AFAICS, the test doesn't execute the compiled object's code, so
>>>> GDB will try to read memory from the binary's sections.  As the
>>>> section is only 1 byte long, and probably no other section is allocated
>>>> contiguously, that'll fail...  To confirm, debug GDB under GDB,
>>>> and put a break on throw_it or some such.  Then work up the stack
>>>> to see where that is thrown, and why.
>>>>
>>>> --
>>>> Pedro Alves
>>>>
>>>
>>> I have verified the error is being thrown by gdb while its unable to
>>> read the 4byte breakpoint address.
>>> Heres the call stack:
>>> Thread [1] (Suspended: Breakpoint hit.)
>>> 38 throw_error() exceptions.c:444 0x0016728c
>>> 37 memory_error() corefile.c:204 0x001d1fcc
>>> 36 read_memory() corefile.c:223 0x001d201a
>>> 35 read_memory_unsigned_integer() corefile.c:312 0x001d2166
>>> 34 arm_skip_prologue() arm-tdep.c:1452 0x00054270
>>
>> Right, though this is actually parsing the prologue:
>>
>> static CORE_ADDR
>> arm_skip_prologue (struct gdbarch *gdbarch, CORE_ADDR pc)
>> {
>> ...
>>   for (skip_pc = pc; skip_pc < limit_pc; skip_pc += 4)
>>     {
>>       inst = read_memory_unsigned_integer (skip_pc, 4, byte_order_for_code);
>>
>> Some ports detect errors and instead return the PC as far
>> as it was managed to be skip.
>> E.g. rs6000-tdep.c:skip_prologue (rs6000==PowerPC):
>>
>>       /* Fetch the instruction and convert it to an integer.  */
>>       if (target_read_memory (pc, buf, 4))
>>       break;
>>       op = extract_unsigned_integer (buf, 4, byte_order);
>>
>> But not all do that.  SPARC also doesn't throw.  But others do throw
>> an error like ARM.  I tried SH and that throws error like ARM;  MIPS
>> and xtensa, from inspection, look like they'll throw but I haven't
>> tried it.  AAarch64 throws like ARM, but that's not surprising.
>> Anyway, there's no standard.
>>
>>> 33 gdbarch_skip_prologue() gdbarch.c:2603 0x00176e5c
>>> 32 skip_prologue_sal() symtab.c:2869 0x0013dad2
>>> 31 find_function_start_sal() symtab.c:2782 0x0013d9aa
>>> 30 symbol_to_sal() linespec.c:3622 0x0014f722
>>> 29 convert_linespec_to_sals() linespec.c:2028 0x0014d6fa
>>> 28 parse_linespec() linespec.c:2319 0x0014dc04
>>> 27 decode_line_full() linespec.c:2430 0x0014df44
>>> 26 parse_breakpoint_sals() breakpoint.c:9323 0x00108560
>> ...
>>
>>> I guess only way to address it is to either use the patch I have
>>> posted or may be disable the test for arm? Any suggestions?
>>
>> Another other way to handle this would be to make the prologue
>> scanner cope with this, and not error out, like some ports do.  But
>> it's not clear at all to me that's a useful behavior.  Even if we
>> pretended we found the end of the prologue in this case, the address
>> we would find in this particular case would never be a valid address
>> to put a breakpoint at (the function's first address).  If we tried
>> setting a breakpoint there, who knows what is it would be overwritten
>> by the bytes that fall off the section (we can be 99.99% sure
>> the next section would be aligned, and the gap wouldn't be used
>> for anything, but still...  So, I think it might be better to leave
>> the scanner as is, throwing the error while it has context about
>> it, and let the user (or higher-level code) decide what to do.
>>
>> Another way to tackle this could be to actually disable prologue
>> skipping, by setting the breakpoint at exactly the func's first
>> instruction, with the '*'/address operator:
>>
>> -gdb_test "break 'z:file.c':func" {Breakpoint [0-9]+ at .*}
>> +gdb_test "break *'z:file.c'::func" {Breakpoint [0-9]+ at .*}
>>
>> This doesn't actually work, though I think that's a bug.  I'll
>> file a PR.
>>
>> But, even if it did, that converts a linespec to an expression,
>> which may not be a universal solution, as tests with this issue
>> might need to use a "real" linespec...
>>
>> So, in the end, it'd be fine with me to just go in the
>> direction of your original patch then.  But I think it deserves
>> a comment:
>>
>>  pc_start:
>>         /* Enough space to fit one instruction.  */
>> -       .byte   0
>> +       .4byte  0
>>  pc_end:
>>
>> Could you resend your patch, with that change, a fixed commit
>> log description and fixed ChangeLog?
>>
>> Thanks,
>>
>
> Sorry about responding late to this. I have attached the patch along with commit message and a ChangeLog.
>
> Commit Log Message:
>
>         Avoid test failure due to error thrown from skip prologue code by
>         an illegal memory access in case of single byte text section
>
> gdb/testsuite/ChangeLog:
>
> 2013-12-02  Omair Javaid  <Omair.Javaid@linaro.org>
>
>         * gdb.dwarf2/dw2-dos-drive.S: Changed text section size to 4 bytes
>
> ---
>  gdb/testsuite/gdb.dwarf2/dw2-dos-drive.S | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/gdb/testsuite/gdb.dwarf2/dw2-dos-drive.S b/gdb/testsuite/gdb.dwarf2/dw2-dos-drive.S
> index 682ba4e..f226912 100644
> --- a/gdb/testsuite/gdb.dwarf2/dw2-dos-drive.S
> +++ b/gdb/testsuite/gdb.dwarf2/dw2-dos-drive.S
> @@ -15,7 +15,7 @@
>
>         .text
>  pc_start:
> -       .byte   0
> +       .4byte  0
>  pc_end:
>
>         .section        .debug_info
> --

Ping! OK to commit?