Hi,
I look forward to using this! :)
I just have afew comments below.
On Mon, Oct 09, 2017 at 05:05:19PM +0200, Alexander Potapenko wrote:
> +/*
> + * Defines the format for the types of collected comparisons.
> + */
> +enum kcov_cmp_type {
> + /*
> + * LSB shows whether one of the arguments is a compile-time constant.
> + */
> + KCOV_CMP_CONST = 1,
> + /*
> + * Second and third LSBs contain the size of arguments (1/2/4/8 bytes).
> + */
> + KCOV_CMP_SIZE1 = 0,
> + KCOV_CMP_SIZE2 = 2,
> + KCOV_CMP_SIZE4 = 4,
> + KCOV_CMP_SIZE8 = 6,
> + KCOV_CMP_SIZE_MASK = 6,
> +};
Given that LSB is meant to be OR-ed in, (and hence combinations of
values are meaningful) I don't think it makes sense for this to be an
enum. This would clearer as something like:
/*
* The format for the types of collected comparisons.
*
* Bit 0 shows whether one of the arguments is a compile-time constant.
* Bits 1 & 2 contain log2 of the argument size, up to 8 bytes.
*/
#define KCOV_CMP_CONST (1 << 0)
#define KCOV_CMP_SIZE(n) ((n) << 1)
#define KCOV_CMP_MASK KCOV_CMP_SIZE(3)
... I note that a few places in the kernel use a 128-bit type. Are
128-bit comparisons not instrumented?
[...]
> +static bool check_kcov_mode(enum kcov_mode needed_mode, struct task_struct *t)
> +{
> + enum kcov_mode mode;
> +
> + /*
> + * We are interested in code coverage as a function of a syscall inputs,
> + * so we ignore code executed in interrupts.
> + */
> + if (!t || !in_task())
> + return false;
This !t check can go, as with the one in __sanitizer_cov_trace_pc, since
t is always current, and therefore cannot be NULL.
IIRC there's a patch queued for that, which this may conflict with.
> + mode = READ_ONCE(t->kcov_mode);
> + /*
> + * There is some code that runs in interrupts but for which
> + * in_interrupt() returns false (e.g. preempt_schedule_irq()).
> + * READ_ONCE()/barrier() effectively provides load-acquire wrt
> + * interrupts, there are paired barrier()/WRITE_ONCE() in
> + * kcov_ioctl_locked().
> + */
> + barrier();
> + if (mode != needed_mode)
> + return false;
> + return true;
This would be simpler as:
return mode == needed_mode;
[...]
> + area = t->kcov_area;
> + /* The first 64-bit word is the number of subsequent PCs. */
> + pos = READ_ONCE(area[0]) + 1;
> + if (likely(pos < t->kcov_size)) {
> + area[pos] = ip;
> + WRITE_ONCE(area[0], pos);
Not a new problem, but if the area for one thread is mmap'd, and read by
another thread, these two writes could be seen out-of-order, since we
don't have an smp_wmb() between them.
I guess Syzkaller doesn't read the mmap'd kcov file from another thread?
> }
> }
> EXPORT_SYMBOL(__sanitizer_cov_trace_pc);
>
> +#ifdef CONFIG_KCOV_ENABLE_COMPARISONS
> +static void write_comp_data(u64 type, u64 arg1, u64 arg2, u64 ip)
> +{
> + struct task_struct *t;
> + u64 *area;
> + u64 count, start_index, end_pos, max_pos;
> +
> + t = current;
> + if (!check_kcov_mode(KCOV_MODE_TRACE_CMP, t))
> + return;
> +
> +#ifdef CONFIG_RANDOMIZE_BASE
> + ip -= kaslr_offset();
> +#endif
Given we have this in two places, it might make sense to have a helper
like:
unsigned long canonicalize_ip(unsigned long ip)
{
#ifdef CONFIG_RANDOMIZE_BASE
ip -= kaslr_offset();
#endif
return ip;
}
... to minimize the ifdeffery elsewhere.
> +
> + /*
> + * We write all comparison arguments and types as u64.
> + * The buffer was allocated for t->kcov_size unsigned longs.
> + */
> + area = (u64 *)t->kcov_area;
> + max_pos = t->kcov_size * sizeof(unsigned long);
> +
> + count = READ_ONCE(area[0]);
> +
> + /* Every record is KCOV_WORDS_PER_CMP 64-bit words. */
> + start_index = 1 + count * KCOV_WORDS_PER_CMP;
> + end_pos = (start_index + KCOV_WORDS_PER_CMP) * sizeof(u64);
> + if (likely(end_pos <= max_pos)) {
> + area[start_index] = type;
> + area[start_index + 1] = arg1;
> + area[start_index + 2] = arg2;
> + area[start_index + 3] = ip;
> + WRITE_ONCE(area[0], count + 1);
That ordering problem applies here, too.
Thanks,
Mark.
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