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

Athira Rajeev atrajeev at linux.vnet.ibm.com
Fri Aug 11 15:21:28 AEST 2023

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.

Signed-off-by: Athira Rajeev <atrajeev at 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 */
+	if (strstr(dso->long_name, ".ko")) {
+		if ((al.addr < map__end(al.map)) && (al.addr > dso->text_end)) {
+			pr_debug(" - skipping\n");
+			goto out;
+		}
+	}
 	/* Read the object code using perf */
 	ret_len = dso__data_read_offset(dso, maps__machine(thread__maps(thread)),
 					al.addr, buf1, len);

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