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

[Athira Rajeev]

> On 14-Sep-2023, at 5:47 PM, Disha Goel <disgoel at linux.ibm.com> wrote:
> 
> On 07/09/23 10:15 pm, 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 ߦ0xc008000007d00000
>> 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 at linux.ibm.com>
>> Signed-off-by: Athira Rajeev <atrajeev at linux.vnet.ibm.com>
>> ---
>> Changelog:
>> v1 -> v2:
>> Updated comment to add description on which arch has stub and
>> reason for skipping as suggested by Adrian

Thanks for testing Disha.

Hi Adrian,

Can you please review and share feedback on this version.

Thanks
Athira

> With this patch applied perf Object code reading test works correctly.
> 
> 26: Object code reading : Ok
> 
> Tested-by: Disha Goel <disgoel at linux.ibm.com>
> 
>> tools/perf/tests/code-reading.c | 12 ++++++++++++
>> 1 file changed, 12 insertions(+)
>> 
>> diff --git a/tools/perf/tests/code-reading.c b/tools/perf/tests/code-reading.c
>> index ed3815163d1b..3cf6c2d42416 100644
>> --- a/tools/perf/tests/code-reading.c
>> +++ b/tools/perf/tests/code-reading.c
>> @@ -269,6 +269,18 @@ 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;
>> 
>> + /*
>> + * Some architectures (ex: powerpc) have stubs (trampolines) in kernel
>> + * modules to manage long jumps. Check if the ip offset falls in stubs
>> + * sections for kernel modules. And skip module address after text end
>> + */
>> + if (strstr(dso->long_name, ".ko")) {
>> + if (al.addr > dso->text_end) {
>> + pr_debug("skipping the module address %#"PRIx64" after text end\n", al.addr);
>> + goto out;
>> + }
>> + }
>> +
>> /* Read the object code using perf */
>> ret_len = dso__data_read_offset(dso, maps__machine(thread__maps(thread)),
>> al.addr, buf1, len);
>>