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.
