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

[Omair Javaid <omair dot javaid at linaro dot org>]

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
--