Re: Another gcc corruption bug (was Re: [PATCH] [RFC] x86: avoid -mtune=atom for objtool warnings)

[Denys Vlasenko <dvlasenk@redhat.com>]

On 10/13/2016 02:46 PM, Josh Poimboeuf wrote:
> On Tue, Oct 11, 2016 at 10:38:42PM +0200, Arnd Bergmann wrote:
>> On Tuesday, October 11, 2016 10:51:46 AM CEST Josh Poimboeuf wrote:
>>> Notice how it just falls off the end of the function.  We had a similar
>>> bug before:
>>>
>>>   https://lkml.kernel.org/r/20160413033649.7r3msnmo3trtq47z@treble
>>
>> I remember that nightmare :(
>>
>>>   https://gcc.gnu.org/bugzilla/show_bug.cgi?id=70646
>>>
>>> I'm not sure yet if this is the same gcc bug or a different one.  Maybe
>>> it's related to the new GCC_PLUGIN_SANCOV?
>>
>> I've reduced one of the test cases to this now:
>>
>> /* gcc-6  -O2 -fno-strict-aliasing -fno-reorder-blocks -fno-omit-frame-pointer  -Wno-pointer-sign -fsanitize-coverage=trace-pc -Wall -Werror -c snic_res.c -o snic_res.o */
>> typedef int spinlock_t;
>> extern unsigned int ioread32(void *);
>> struct vnic_wq_ctrl {
>> 	unsigned int error_status;
>> };
>> struct vnic_wq {
>> 	struct vnic_wq_ctrl *ctrl;
>> } mempool_t;
>> struct snic {
>> 	unsigned int wq_count;
>> 	__attribute__ ((__aligned__)) struct vnic_wq wq[1];
>> 	spinlock_t wq_lock[1];
>> };
>> unsigned int snic_log_q_error_err_status;
>> void snic_log_q_error(struct snic *snic)
>> {
>> 	unsigned int i;
>> 	for (i = 0; i < snic->wq_count; i++)
>> 		snic_log_q_error_err_status =
>> 		    ioread32(&snic->wq[i].ctrl->error_status);
>> }
>>
>> which gets compiled into
>>
>> 0000000000000000 <snic_log_q_error>:
>>    0:	55                   	push   %rbp
>>    1:	48 89 e5             	mov    %rsp,%rbp
>>    4:	53                   	push   %rbx
>>    5:	48 89 fb             	mov    %rdi,%rbx
>>    8:	48 83 ec 08          	sub    $0x8,%rsp
>>    c:	e8 00 00 00 00       	callq  11 <snic_log_q_error+0x11>
>> 			d: R_X86_64_PC32	__sanitizer_cov_trace_pc-0x4
>>   11:	8b 03                	mov    (%rbx),%eax
>>   13:	85 c0                	test   %eax,%eax
>>   15:	75 11                	jne    28 <snic_log_q_error+0x28>
>>   17:	48 83 c4 08          	add    $0x8,%rsp
>>   1b:	5b                   	pop    %rbx
>>   1c:	5d                   	pop    %rbp
>>   1d:	e9 00 00 00 00       	jmpq   22 <snic_log_q_error+0x22>
>> 			1e: R_X86_64_PC32	__sanitizer_cov_trace_pc-0x4
>>   22:	66 0f 1f 44 00 00    	nopw   0x0(%rax,%rax,1)
>>   28:	e8 00 00 00 00       	callq  2d <snic_log_q_error+0x2d>
>> 			29: R_X86_64_PC32	__sanitizer_cov_trace_pc-0x4
>>   2d:	48 8b 7b 10          	mov    0x10(%rbx),%rdi
>>   31:	e8 00 00 00 00       	callq  36 <snic_log_q_error+0x36>
>> 			32: R_X86_64_PC32	ioread32-0x4
>>   36:	89 05 00 00 00 00    	mov    %eax,0x0(%rip)        # 3c <snic_log_q_error+0x3c>
>> 			38: R_X86_64_PC32	snic_log_q_error_err_status-0x4
>>   3c:	83 3b 01             	cmpl   $0x1,(%rbx)
>>   3f:	76 d6                	jbe    17 <snic_log_q_error+0x17>
>>   41:	e8 00 00 00 00       	callq  46 <snic_log_q_error+0x46>
>> 			42: R_X86_64_PC32	__sanitizer_cov_trace_pc-0x4
>
> I opened a bug:
>
>   https://gcc.gnu.org/bugzilla/show_bug.cgi?id=77966
>

Surprisingly, it's really "not a bug". The only way you can end up in this branch
is if you have a bug and run off the end of wq[1] array member: i.e.
if snic->wq_count >= 2. (See gcc BZ for smaller example)

It's debatable whether it's okay for gcc to just let buggy code to run off
and execute something random. It is surely surprising, and not debug-friendly.

An option to emit a crashing instruction (HLT, INT3, that sort of thing)
instead of just stopping code generation might be useful.