Re: [PATCH V2 2/2] tools/perf/tests: Fix object code reading to skip address that falls out of text section

[Athira Rajeev <atrajeev@linux.vnet.ibm.com>]

> On 18-Aug-2023, at 12:45 PM, Adrian Hunter <adrian.hunter@intel.com> wrote:
> 
> On 17/08/23 20:18, Athira Rajeev wrote:
>> The testcase "Object code reading" fails in somecases
>> for "fs_something" sub test as below:
>> 
>>    Reading object code for memory address: 0xc008000007f0142c
>>    File is: /lib/modules/6.5.0-rc3+/kernel/fs/xfs/xfs.ko
>>    On file address is: 0x1114cc
>>    Objdump command is: objdump -z -d --start-address=0x11142c --stop-address=0x1114ac /lib/modules/6.5.0-rc3+/kernel/fs/xfs/xfs.ko
>>    objdump read too few bytes: 128
>>    test child finished with -1
>> 
>> This can alo be reproduced when running perf record with
>> workload that exercises fs_something() code. In the test
>> setup, this is exercising xfs code since root is xfs.
>> 
>>    # perf record ./a.out
>>    # perf report -v |grep "xfs.ko"
>>      0.76% a.out /lib/modules/6.5.0-rc3+/kernel/fs/xfs/xfs.ko  0xc008000007de5efc B [k] xlog_cil_commit
>>      0.74% a.out  /lib/modules/6.5.0-rc3+/kernel/fs/xfs/xfs.ko  0xc008000007d5ae18 B [k] xfs_btree_key_offset
>>      0.74% a.out  /lib/modules/6.5.0-rc3+/kernel/fs/xfs/xfs.ko  0xc008000007e11fd4 B [k] 0x0000000000112074
>> 
>> Here addr "0xc008000007e11fd4" is not resolved. since this is a
>> kernel module, its offset is from the DSO. Xfs module is loaded
>> at 0xc008000007d00000
>> 
>>   # cat /proc/modules | grep xfs
>>    xfs 2228224 3 - Live 0xc008000007d00000
>> 
>> And size is 0x220000. So its loaded between  0xc008000007d00000
>> and 0xc008000007f20000. From objdump, text section is:
>>    text 0010f7bc  0000000000000000 0000000000000000 000000a0 2**4
>> 
>> Hence perf captured ip maps to 0x112074 which is:
>> ( ip - start of module ) + a0
>> 
>> This offset 0x112074 falls out .text section which is up to 0x10f7bc
>> In this case for module, the address 0xc008000007e11fd4 is pointing
>> to stub instructions. This address range represents the module stubs
>> which is allocated on module load and hence is not part of DSO offset.
>> 
>> To address this issue in "object code reading", skip the sample if
>> address falls out of text section and is within the module end.
>> Use the "text_end" member of "struct dso" to do this check.
>> 
>> To address this issue in "perf report", exploring an option of
>> having stubs range as part of the /proc/kallsyms, so that perf
>> report can resolve addresses in stubs range
>> 
>> However this patch uses text_end to skip the stub range for
>> Object code reading testcase.
>> 
>> Reported-by: Disha Goel <disgoel@linux.ibm.com>
>> Signed-off-by: Athira Rajeev <atrajeev@linux.vnet.ibm.com>
>> ---
>> tools/perf/tests/code-reading.c | 8 ++++++++
>> 1 file changed, 8 insertions(+)
>> 
>> diff --git a/tools/perf/tests/code-reading.c b/tools/perf/tests/code-reading.c
>> index ed3815163d1b..911f8fa13677 100644
>> --- a/tools/perf/tests/code-reading.c
>> +++ b/tools/perf/tests/code-reading.c
>> @@ -269,6 +269,14 @@ static int read_object_code(u64 addr, size_t len, u8 cpumode,
>> if (addr + len > map__end(al.map))
>> len = map__end(al.map) - addr;
>> 
>> + /* Check if the ip offset falls in stubs sections for kernel modules */
> 
> What arches have stubs - maybe expand the comment a bit e.g.
> "Some architectures (such as blah blah) have stubs (trampolines) in kernel modules to manage long jumps”
Ok
> 
>> + if (strstr(dso->long_name, ".ko")) {
>> + if ((al.addr < map__end(al.map)) && (al.addr > dso->text_end)) {
> 
> Why check al.addr < map__end(al.map) ?  addr must be on the map mustn't it?

Yes, you are right. 
> 
> Also please remove redundant parentheses.
> 
>> + pr_debug(" - skipping\n");
> 
> "skipping" but why.  Maybe "skipping module address after text end"

Sure, will post V2 with these changes

Thanks
Athira
> 
>> + goto out;
>> + }
>> + }
>> +
>> /* Read the object code using perf */
>> ret_len = dso__data_read_offset(dso, maps__machine(thread__maps(thread)),
>> al.addr, buf1, len);