Re: [RFC PATCH v1] powerpc: Enable KFENCE for PPC32

Christophe Leroy
2021-03-05T08:23:58Z

Le 05/03/2021 à 08:50, Marco Elver a écrit :
> On Fri, Mar 05, 2021 at 04:01PM +1100, Michael Ellerman wrote:
>> Marco Elver <elver@google.com> writes:
>>> On Thu, Mar 04, 2021 at 12:48PM +0100, Christophe Leroy wrote:
>>>> Le 04/03/2021 à 12:31, Marco Elver a écrit :
>>>>> On Thu, 4 Mar 2021 at 12:23, Christophe Leroy
>>>>> <christophe.leroy@csgroup.eu> wrote:
>>>>>> Le 03/03/2021 à 11:56, Marco Elver a écrit :
>>>>>>>
>>>>>>> Somewhat tangentially, I also note that e.g. show_regs(regs) (which
>>>>>>> was printed along the KFENCE report above) didn't include the top
>>>>>>> frame in the "Call Trace", so this assumption is definitely not
>>>>>>> isolated to KFENCE.
>>>>>>>
>>>>>>
>>>>>> Now, I have tested PPC64 (with the patch I sent yesterday to modify save_stack_trace_regs()
>>>>>> applied), and I get many failures. Any idea ?
>>>>>>
>>>>>> [ 17.653751][ T58] ==================================================================
>>>>>> [ 17.654379][ T58] BUG: KFENCE: invalid free in .kfence_guarded_free+0x2e4/0x530
>>>>>> [ 17.654379][ T58]
>>>>>> [ 17.654831][ T58] Invalid free of 0xc00000003c9c0000 (in kfence-#77):
>>>>>> [ 17.655358][ T58] .kfence_guarded_free+0x2e4/0x530
>>>>>> [ 17.655775][ T58] .__slab_free+0x320/0x5a0
>>>>>> [ 17.656039][ T58] .test_double_free+0xe0/0x198
>>>>>> [ 17.656308][ T58] .kunit_try_run_case+0x80/0x110
>>>>>> [ 17.656523][ T58] .kunit_generic_run_threadfn_adapter+0x38/0x50
>>>>>> [ 17.657161][ T58] .kthread+0x18c/0x1a0
>>>>>> [ 17.659148][ T58] .ret_from_kernel_thread+0x58/0x70
>>>>>> [ 17.659869][ T58]
>>> [...]
>>>>>
>>>>> Looks like something is prepending '.' to function names. We expect
>>>>> the function name to appear as-is, e.g. "kfence_guarded_free",
>>>>> "test_double_free", etc.
>>>>>
>>>>> Is there something special on ppc64, where the '.' is some convention?
>>>>>
>>>>
>>>> I think so, see https://refspecs.linuxfoundation.org/ELF/ppc64/PPC-elf64abi.html#FUNC-DES
>>>>
>>>> Also see commit https://github.com/linuxppc/linux/commit/02424d896
>>>
>>> Thanks -- could you try the below patch? You'll need to define
>>> ARCH_FUNC_PREFIX accordingly.
>>>
>>> We think, since there are only very few architectures that add a prefix,
>>> requiring <asm/kfence.h> to define something like ARCH_FUNC_PREFIX is
>>> the simplest option. Let me know if this works for you.
>>>
>>> There an alternative option, which is to dynamically figure out the
>>> prefix, but if this simpler option is fine with you, we'd prefer it.
>>
>> We have rediscovered this problem in basically every tracing / debugging
>> feature added in the last 20 years :)
>>
>> I think the simplest solution is the one tools/perf/util/symbol.c uses,
>> which is to just skip a leading '.'.
>>
>> Does that work?
>>
>> diff --git a/mm/kfence/report.c b/mm/kfence/report.c
>> index ab83d5a59bb1..67b49dc54b38 100644
>> --- a/mm/kfence/report.c
>> +++ b/mm/kfence/report.c
>> @@ -67,6 +67,9 @@ static int get_stack_skipnr(const unsigned long stack_entries[], int num_entries
>> for (skipnr = 0; skipnr < num_entries; skipnr++) {
>> int len = scnprintf(buf, sizeof(buf), "%ps", (void *)stack_entries[skipnr]);
>>
>> + if (buf[0] == '.')
>> + buf++;
>> +
>
> Unfortunately this does not work, since buf is an array. We'd need an
> offset, and it should be determined outside the loop. I had a solution
> like this, but it turned out quite complex (see below). And since most
> architectures do not require this, decided that the safest option is to
> use the macro approach with ARCH_FUNC_PREFIX, for which Christophe
> already prepared a patch and tested:
> https://lore.kernel.org/linux-mm/20210304144000.1148590-1-elver@google.com/
> https://lkml.kernel.org/r/afaec81a551ef15345cb7d7563b3fac3d7041c3a.1614868445.git.christophe.leroy@csgroup.eu
>
> Since KFENCE requires <asm/kfence.h> anyway, we'd prefer this approach
> (vs. dynamically detecting).
>
> Thanks,
> -- Marco
>

What about

diff --git a/mm/kfence/report.c b/mm/kfence/report.c
index 519f037720f5..5e196625fb34 100644
--- a/mm/kfence/report.c
+++ b/mm/kfence/report.c
@@ -43,7 +43,7 @@ static void seq_con_printf(struct seq_file *seq, const char *fmt, ...)
static int get_stack_skipnr(const unsigned long stack_entries[], int num_entries,
const enum kfence_error_type *type)
{
- char buf[64];
+ char _buf[64];
int skipnr, fallback = 0;

if (type) {
@@ -65,7 +65,11 @@ static int get_stack_skipnr(const unsigned long stack_entries[], int num_entries
}

for (skipnr = 0; skipnr < num_entries; skipnr++) {
- int len = scnprintf(buf, sizeof(buf), "%ps", (void *)stack_entries[skipnr]);
+ char *buf = _buf;
+ int len = scnprintf(_buf, sizeof(_buf), "%ps", (void *)stack_entries[skipnr]);
+
+ if (_buf[0] == '.')
+ buf++, len--;

if (str_has_prefix(buf, "kfence_") || str_has_prefix(buf, "__kfence_") ||
!strncmp(buf, "__slab_free", len)) {
---

Christophe