Re: [PATCH 13/15] powerpc/watchpoint: Don't allow concurrent perf and ptrace events

[Ravi Bangoria <ravi.bangoria@linux.ibm.com>]

On 3/17/20 4:38 PM, Christophe Leroy wrote:
> 
> 
> Le 09/03/2020 à 09:58, Ravi Bangoria a écrit :
>> ptrace and perf watchpoints on powerpc behaves differently. Ptrace
> 
> On the 8xx, ptrace generates signal after executing the instruction.

8xx logic is unchanged. I should have mentioned "Book3s DAWR".

> 
>> watchpoint works in one-shot mode and generates signal before executing
>> instruction. It's ptrace user's job to single-step the instruction and
>> re-enable the watchpoint. OTOH, in case of perf watchpoint, kernel
>> emulates/single-steps the instruction and then generates event. If perf
>> and ptrace creates two events with same or overlapping address ranges,
>> it's ambiguous to decide who should single-step the instruction. Because
>> of this issue ptrace and perf event can't coexist when the address range
>> overlaps.
> 
> Ok, and then ? What's the purpose of this (big) patch ?

Don't allow perf and ptrace watchpoint at the same time if their address
range overlaps.

...

>> +struct breakpoint {
>> +    struct list_head list;
>> +    struct perf_event *bp;
>> +    bool ptrace_bp;
>> +};
> 
> Don't we have an equivalent struct already ?

No. Using this we track percpu and perthread watchpoints for both perf and
ptrace. This problems is powerpc(DAWR) specific and thus we need to hook arch
specific logic in watchopint installation/uninstallation path.

...

>> +static bool bp_addr_range_overlap(struct perf_event *bp1, struct perf_event *bp2)
>> +{
>> +    __u64 bp1_saddr, bp1_eaddr, bp2_saddr, bp2_eaddr;
>> +
>> +    bp1_saddr = bp1->attr.bp_addr & ~HW_BREAKPOINT_ALIGN;
>> +    bp1_eaddr = (bp1->attr.bp_addr + bp1->attr.bp_len - 1) | HW_BREAKPOINT_ALIGN;
>> +    bp2_saddr = bp2->attr.bp_addr & ~HW_BREAKPOINT_ALIGN;
>> +    bp2_eaddr = (bp2->attr.bp_addr + bp2->attr.bp_len - 1) | HW_BREAKPOINT_ALIGN;
>> +
>> +    return (bp1_saddr <= bp2_eaddr && bp1_eaddr >= bp2_saddr);
> 
> Would be better with something like (HW_BREAKPOINT_SIZE needs to be defined).
> 
>      bp1_saddr = ALIGN_DOWN(bp1->attr.bp_addr, HW_BREAKPOINT_SIZE);
>      bp1_eaddr = ALIGN(bp1->attr.bp_addr, HW_BREAKPOINT_SIZE);
>      bp2_saddr = ALIGN_DOWN(bp2->attr.bp_addr, HW_BREAKPOINT_SIZE);
>      bp2_eaddr = ALIGN(bp2->attr.bp_addr, HW_BREAKPOINT_SIZE);
> 
>      return (bp1_saddr < bp2_eaddr && bp1_eaddr > bp2_saddr);

Ok.

Thanks,
Ravi