Re: [PATCH v2] perf probe: fix kretprobe issue caused by GCC bug

Arnaldo Carvalho de Melo
2021-02-12T21:35:23Z

Em Wed, Feb 10, 2021 at 02:26:46PM +0800, Jianlin Lv escreveu:
> Perf failed to add kretprobe event with debuginfo of vmlinux which is
> compiled by gcc with -fpatchable-function-entry option enabled.
> The same issue with kernel module.
>
> Issue:
>
> # perf probe -v 'kernel_clone%return $retval'
> ......
> Writing event: r:probe/kernel_clone__return _text+599624 $retval
> Failed to write event: Invalid argument
> Error: Failed to add events. Reason: Invalid argument (Code: -22)
>
> # cat /sys/kernel/debug/tracing/error_log
> [156.75] trace_kprobe: error: Retprobe address must be an function entry
> Command: r:probe/kernel_clone__return _text+599624 $retval
> ^
>
> # llvm-dwarfdump vmlinux |grep -A 10 -w 0x00df2c2b
> 0x00df2c2b: DW_TAG_subprogram
> DW_AT_external (true)
> DW_AT_name ("kernel_clone")
> DW_AT_decl_file ("/home/code/linux-next/kernel/fork.c")
> DW_AT_decl_line (2423)
> DW_AT_decl_column (0x07)
> DW_AT_prototyped (true)
> DW_AT_type (0x00dcd492 "pid_t")
> DW_AT_low_pc (0xffff800010092648)
> DW_AT_high_pc (0xffff800010092b9c)
> DW_AT_frame_base (DW_OP_call_frame_cfa)
>
> # cat /proc/kallsyms |grep kernel_clone
> ffff800010092640 T kernel_clone
> # readelf -s vmlinux |grep -i kernel_clone
> 183173: ffff800010092640 1372 FUNC GLOBAL DEFAULT 2 kernel_clone
>
> # objdump -d vmlinux |grep -A 10 -w \<kernel_clone\>:
> ffff800010092640 <kernel_clone>:
> ffff800010092640: d503201f nop
> ffff800010092644: d503201f nop
> ffff800010092648: d503233f paciasp
> ffff80001009264c: a9b87bfd stp x29, x30, [sp, #-128]!
> ffff800010092650: 910003fd mov x29, sp
> ffff800010092654: a90153f3 stp x19, x20, [sp, #16]
>
> The entry address of kernel_clone converted by debuginfo is _text+599624
> (0x92648), which is consistent with the value of DW_AT_low_pc attribute.
> But the symbolic address of kernel_clone from /proc/kallsyms is
> ffff800010092640.
>
> This issue is found on arm64, -fpatchable-function-entry=2 is enabled when
> CONFIG_DYNAMIC_FTRACE_WITH_REGS=y;
> Just as objdump displayed the assembler contents of kernel_clone,
> GCC generate 2 NOPs at the beginning of each function.
>
> kprobe_on_func_entry detects that (_text+599624) is not the entry address
> of the function, which leads to the failure of adding kretprobe event.
>
> ---
> kprobe_on_func_entry
> ->_kprobe_addr
> ->kallsyms_lookup_size_offset
> ->arch_kprobe_on_func_entry // FALSE
> ---

Please don't use --- at the start of a line, it is used to separate from
the patch itself, later down your message.

It causes this:

[acme@five perf]$ am /wb/1.patch
Traceback (most recent call last):
File "/home/acme/bin/ksoff.py", line 180, in <module>
sign_msg(sys.stdin, sys.stdout)
File "/home/acme/bin/ksoff.py", line 142, in sign_msg
sob.remove(last_sob[0])
TypeError: 'NoneType' object is not subscriptable
[acme@five perf]$

I'm fixing this by removing that --- markers

> The cause of the issue is that the first instruction in the compile unit
> indicated by DW_AT_low_pc does not include NOPs.
> This issue exists in all gcc versions that support
> -fpatchable-function-entry option.
>
> I have reported it to the GCC community:
> https://gcc.gnu.org/bugzilla/show_bug.cgi?id=98776
>
> Currently arm64 and PA-RISC may enable fpatchable-function-entry option.
> The kernel compiled with clang does not have this issue.
>
> FIX:
>
> This GCC issue only cause the registration failure of the kretprobe event
> which doesn't need debuginfo. So, stop using debuginfo for retprobe.
> map will be used to query the probe function address.
>
> Signed-off-by: Jianlin Lv <Jianlin.Lv@arm.com>
> ---
> v2: stop using debuginfo for retprobe, and update changelog.
> ---
> tools/perf/util/probe-event.c | 10 ++++++++++
> 1 file changed, 10 insertions(+)
>
> diff --git a/tools/perf/util/probe-event.c b/tools/perf/util/probe-event.c
> index 8eae2afff71a..a59d3268adb0 100644
> --- a/tools/perf/util/probe-event.c
> +++ b/tools/perf/util/probe-event.c
> @@ -894,6 +894,16 @@ static int try_to_find_probe_trace_events(struct perf_probe_event *pev,
> struct debuginfo *dinfo;
> int ntevs, ret = 0;
>
> + /* Workaround for gcc #98776 issue.
> + * Perf failed to add kretprobe event with debuginfo of vmlinux which is
> + * compiled by gcc with -fpatchable-function-entry option enabled. The
> + * same issue with kernel module. The retprobe doesn`t need debuginfo.
> + * This workaround solution use map to query the probe function address
> + * for retprobe event.
> + */
> + if (pev->point.retprobe)
> + return 0;
> +
> dinfo = open_debuginfo(pev->target, pev->nsi, !need_dwarf);
> if (!dinfo) {
> if (need_dwarf)
> --
> 2.25.1
>

--

- Arnaldo