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

[Josh Poimboeuf]

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

-- 
Josh