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From:   Jianlin Lv <Jianlin.Lv@arm.com>
To:     peterz@infradead.org, mingo@redhat.com, acme@kernel.org,
        mark.rutland@arm.com, alexander.shishkin@linux.intel.com,
        jolsa@redhat.com, namhyung@kernel.org, nathan@kernel.org,
        ndesaulniers@google.com, mhiramat@kernel.org, fche@redhat.com,
        irogers@google.com, sumanthk@linux.ibm.com
Cc:     Jianlin.Lv@arm.com, linux-kernel@vger.kernel.org,
        clang-built-linux@googlegroups.com
Subject: [PATCH v2] perf probe: fix kretprobe issue caused by GCC bug
Date:   Wed, 10 Feb 2021 14:26:46 +0800
Message-Id: <20210210062646.2377995-1-Jianlin.Lv@arm.com>
MIME-Version: 1.0
Content-Transfer-Encoding: 8bit
Precedence: bulk

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
---

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