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

[Adrian Hunter]

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 at linux.ibm.com>
> 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 */

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"

> +	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?

Also please remove redundant parentheses.

> +			pr_debug(" - skipping\n");

"skipping" but why.  Maybe "skipping module address after text end"

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