Message-ID: <AM6PR08MB3589E9CB66C12943CC93D559988E9@AM6PR08MB3589.eurprd08.prod.outlook.com>
Date:   Tue, 9 Feb 2021 02:51:31 +0000
From:   Jianlin Lv <Jianlin.Lv@....com>
To:     Masami Hiramatsu <mhiramat@...nel.org>
CC:     "peterz@...radead.org" <peterz@...radead.org>,
        "mingo@...hat.com" <mingo@...hat.com>,
        "acme@...nel.org" <acme@...nel.org>,
        Mark Rutland <Mark.Rutland@....com>,
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        <clang-built-linux@...glegroups.com>
Subject: RE: [PATCH] perf probe: fix kretprobe issue caused by GCC bug



> -----Original Message-----
> From: Masami Hiramatsu <mhiramat@...nel.org>
> Sent: Monday, February 8, 2021 8:33 PM
> To: Jianlin Lv <Jianlin.Lv@....com>
> Cc: peterz@...radead.org; mingo@...hat.com; acme@...nel.org; Mark
> Rutland <Mark.Rutland@....com>; alexander.shishkin@...ux.intel.com;
> jolsa@...hat.com; namhyung@...nel.org; natechancellor@...il.com;
> ndesaulniers@...gle.com; fche@...hat.com; irogers@...gle.com;
> sumanthk@...ux.ibm.com; adrian.hunter@...el.com; linux-
> kernel@...r.kernel.org; clang-built-linux@...glegroups.com
> Subject: Re: [PATCH] perf probe: fix kretprobe issue caused by GCC bug
>
> Hi Jianlin,
>
> On Fri,  5 Feb 2021 17:35:58 +0800
> Jianlin Lv <Jianlin.Lv@....com> wrote:
>
> > 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.
>
> Oh, I had faced similar bug for fentry.
> 3d918a12a1b3 ("perf probe: Find fentry mcount fuzzed parameter location")
> GCC dwarf generator tends to skip this kind of function entry information...
>
> >
> > 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
>
> Thanks for reporting it!
>
> > 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.
>
> Hmm, it can affects the perf probe with local variables with ftrace
> infrastructure.
>
> Now the debuginfo (dwarf_entrypc(DIE)) will return the actual symbol
> address
> +offset (offset depends on -fpatchable-function-entry). In this case,
> if perf-probe put a probe on a function entry, it will be a bit shifted.
> So, the probe always uses SW break instead of ftrace...Ah, ok...I recalled.
> Before discussing it, I need to restart the kprobe on ftrace for arm64.
> It has been discussed last year, but stopped.

Is there any channel to learn about the progress of restart discussion?
Very interested in this discussion.

>
> > 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.
>
> Hmm, actually, the return probe doesn't need debuginfo since it has no
> information of the local variables when the function returns (of course
> usually all local variables are gone at that point). In that case you can just
> stop using debuginfo for return probe.
> (for the future work, it should support recording the contents of  "pointer
> passing" arguments at return probe, but currently it is not  supported yet. So
> this must be done in another series.)
>
> e.g.
> $ ./perf probe -D "eventfd_signal%return ctx->count"
> Semantic error :You can't specify local variable for kretprobe.
>
> So, this should work.
>
> diff --git a/tools/perf/util/probe-event.c b/tools/perf/util/probe-event.c
> index 8eae2afff71a..10c88885dcd4 100644
> --- a/tools/perf/util/probe-event.c
> +++ b/tools/perf/util/probe-event.c
> @@ -894,6 +894,9 @@ static int try_to_find_probe_trace_events(struct
> perf_probe_event *pev,
>  struct debuginfo *dinfo;
>  int ntevs, ret = 0;
>
> +if (pev->point.retprobe)
> +return 0;
> +
>  dinfo = open_debuginfo(pev->target, pev->nsi, !need_dwarf);
>  if (!dinfo) {
>  if (need_dwarf)
>
> Thank you,
>
>

You are right. I once thought about a similar modification method,
but it felt a little rough, so I added a check for the
"-fpatchable-function-entry" option.

I want a double confirmation, your opinion is to update this patch
as shown above, right?

Jianlin

> >
> > -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@....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
> >
>
>
> --
> Masami Hiramatsu <mhiramat@...nel.org>
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