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Subject[PATCH] perf probe: fix kretprobe issue caused by GCC bug
Date
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:

The result of my investigation is that this GCC issue will only cause the
registration failure of the kretprobe event;
Other functions of perf probe will not be affected, such as line probe,
local variable probe, uprobe, etc.

A workaround solution is to traverse all the compilation units in
debuginfo for the retprobe event and check whether the DW_AT_producer
attribute valaue of each CUs contains substrings: "GNU" and
"-fpatchable-function-entry". If these two substrings are included,
then debuginfo will not be used to convert perf_probe_event.
Instead, map will be used to query the probe function address.

-grecord-gcc-switches causes the command-line options used to invoke the
compiler to be appended to the DW_AT_producer attribute in DWARF debugging
information.It is enabled by default.

A potential defect is that if -gno-record-gcc-switches option is enabled,
the command-line options will not be recorded in debuginfo. This workaround
solution will fail.
Assume that this situation may not happen for kernel compilation.

Signed-off-by: Jianlin Lv <Jianlin.Lv@arm.com>
---
tools/perf/util/probe-event.c | 60 +++++++++++++++++++++++++++++++++++
1 file changed, 60 insertions(+)

diff --git a/tools/perf/util/probe-event.c b/tools/perf/util/probe-event.c
index 8eae2afff71a..c0c1bcc59250 100644
--- a/tools/perf/util/probe-event.c
+++ b/tools/perf/util/probe-event.c
@@ -885,6 +885,60 @@ static int post_process_probe_trace_events(struct perf_probe_event *pev,
return ret;
}

+/*
+ * 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. Refer to gcc issue: #98776
+ * This issue only cause the registration failure of kretprobe event,
+ * and it doesn't affect other perf probe functions.
+ * This workaround solution use map to query the probe function address
+ * for retprobe event.
+ * A potential defect is that if -gno-record-gcc-switches option is enabled,
+ * the command-line options will not be recorded in debuginfo. This workaround
+ * solution will fail.
+ */
+static bool retprobe_gcc_fpatchable_issue_workaround(struct debuginfo *dbg,
+ struct perf_probe_event *pev)
+{
+ Dwarf_Off off = 0, noff = 0;
+ size_t cuhl;
+ Dwarf_Die cu_die;
+ const char *producer = NULL;
+ Dwarf_Attribute attr;
+
+ if (!pev->point.retprobe)
+ return false;
+
+ /* Loop on CUs (Compilation Unit) */
+ while (!dwarf_nextcu(dbg->dbg, off, &noff, &cuhl, NULL, NULL, NULL)) {
+ /* Get the DIE(Debugging Information Entry) of this CU */
+ if (dwarf_offdie(dbg->dbg, off + cuhl, &cu_die) == NULL) {
+ off = noff;
+ continue;
+ }
+
+ /* Get information about the compiler that produced CUs */
+ if (dwarf_hasattr(&cu_die, DW_AT_producer)
+ && dwarf_attr(&cu_die, DW_AT_producer, &attr)) {
+ producer = dwarf_formstring(&attr);
+ if (producer == NULL) {
+ off = noff;
+ continue;
+ }
+ /* Check that CU is compiled by GCC with
+ * fpatchable-function-entry option enabled
+ */
+ if (strstr(producer, "GNU") &&
+ strstr(producer, "-fpatchable-function-entry")) {
+ pr_debug("Workaround for gcc issue, find probe function addresses from map.\n");
+ return true;
+ }
+ }
+ off = noff;
+ }
+ return false;
+}
+
/* Try to find perf_probe_event with debuginfo */
static int try_to_find_probe_trace_events(struct perf_probe_event *pev,
struct probe_trace_event **tevs)
@@ -902,6 +956,12 @@ static int try_to_find_probe_trace_events(struct perf_probe_event *pev,
return 0;
}

+ /* workaround for gcc #98776 issue */
+ if (retprobe_gcc_fpatchable_issue_workaround(dinfo, pev) && !need_dwarf) {
+ debuginfo__delete(dinfo);
+ return 0;
+ }
+
pr_debug("Try to find probe point from debuginfo.\n");
/* Searching trace events corresponding to a probe event */
ntevs = debuginfo__find_trace_events(dinfo, pev, tevs);
--
2.25.1
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