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From: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Date: Fri, 20 Apr 2018 10:10:32 +0200
Message-ID: <CAKv+Gu_sY+Q+kqO1hp6f30WSGNPY1cnW5VJYj9cuVA0hU5YDVw@mail.gmail.com>
Subject: Re: [PATCH] perf tools: set kernel end address properly
To: Kim Phillips <kim.phillips@arm.com>
Cc: Namhyung Kim <namhyung@kernel.org>, Will Deacon <will.deacon@arm.com>,
        Arnaldo Carvalho de Melo <acme@kernel.org>,
        Jiri Olsa <jolsa@redhat.com>, Ingo Molnar <mingo@kernel.org>,
        Peter Zijlstra <a.p.zijlstra@chello.nl>, Jiri Olsa <jolsa@kernel.org>,
        LKML <linux-kernel@vger.kernel.org>, kernel-team@lge.com
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On 20 April 2018 at 01:33, Kim Phillips <kim.phillips@arm.com> wrote:
> On Thu, 19 Apr 2018 11:54:24 +0900
> Namhyung Kim <namhyung@kernel.org> wrote:
>
>> On Wed, Apr 18, 2018 at 07:37:59PM -0500, Kim Phillips wrote:
>> > diff --git a/tools/perf/arch/arm64/util/sym-handling.c b/tools/perf/arch/arm64/util/sym-handling.c
>> > index 0051b1ee8450..5c4a2e208bbc 100644
>> > --- a/tools/perf/arch/arm64/util/sym-handling.c
>> > +++ b/tools/perf/arch/arm64/util/sym-handling.c
>> > @@ -20,3 +20,16 @@ bool elf__needs_adjust_symbols(GElf_Ehdr ehdr)
>> >                ehdr.e_type == ET_DYN;
>> >  }
>> >  #endif
>> > +
>> > +const char *arch__normalize_symbol_name(const char *name)
>> > +{
>> > +       /*
>> > +        * arm64 kernels compensating for a CPU erratum can put up a
>> > +        * module_emit_adrp_veneer in place of a module_emit_plt_entry
>> > +        */
>> > +       if (name && strlen(name) >= 23 &&
>> > +           !strncmp(name, "module_emit_adrp_veneer", 23))
>> > +               return "module_emit_plt_entry";
>> > +
>> > +       return name;
>> > +}
>>
>> I don't know it's always preferable or just for the test.  It it's the
>> latter it may be better to move it to the test code.
>
> AFACT, the veneer is a moniker and doesn't technically exist, and
> shouldn't be being looked-up.  Both chunks of this diff are needed to
> pass perf test 1: this chunk above is because in
> arch__normalize_symbol_name(), we squash the perf test 1's "<veneer>
> not in *kallsyms*" problem, and in the below chunk, we prevent it
> coming up when the test code iterates over the *vmlinux* symbols. I.e.
> we need to prevent the veneer from coming up in both kallsyms *and*
> vmlinux.
>

I don't have all the context here, so I don't know what exactly
arch__normalize_symbol_name() is trying to accomplish.

What I do know is that module_emit_adrp_veneer() and
module_emit_plt_entry() are not part of the veneer themselves: they
are ordinary routines that are part of the module loader, and which
populate the allocated veneer space on demand when encountering ADRP
instructions that need to be rerouted.

>> > diff --git a/tools/perf/tests/vmlinux-kallsyms.c b/tools/perf/tests/vmlinux-kallsyms.c
>> > index 1e5adb65632a..07064e76947d 100644
>> > --- a/tools/perf/tests/vmlinux-kallsyms.c
>> > +++ b/tools/perf/tests/vmlinux-kallsyms.c
>> > @@ -163,6 +163,29 @@ int test__vmlinux_matches_kallsyms(struct test *test __maybe_unused, int subtest
>> >
>> >                                 continue;
>> >                         }
>> > +               } else if (pair) {
>> > +                       s64 skew = mem_start - UM(pair->start);
>> > +                       struct map *kmap = map_groups__find(&kallsyms.kmaps, type, mem_start);
>> > +                       struct map *vmap = map_groups__find(&vmlinux.kmaps, type, mem_start);
>> > +
>> > +                       /*
>> > +                        * arm64 kernels compensating for a CPU erratum can put up a
>> > +                        * module_emit_adrp_veneer in place of a module_emit_plt_entry
>> > +                        */
>> > +                       if (llabs(skew) < page_size)
>>
>> It seems that we needs to check it's the ARM64 at least.  If it's a
>
> OK.
>
>> rare case we might need to add more paranoid checks.
>
> It's certainly rare: Adding the authors of the veneer to cc for
> comments:
>
> Will, Ard, how probable are veneer-style symbols such as the
> one introduced in commit a257e0257 "arm64/kernel: don't ban ADRP to
> work around Cortex-A53 erratum #843419" to happen again in the future?
>

Distro kernels typically enable full KASLR, so on systems that
implement EFI_RNG_PROTOCOL, all function calls from modules into the
kernel proper are redirected via veneers. (Note that these are bl
instructions not adrp instructions though).

> I would have thought WARNing on within-a-pagesize would be OK,
> Namhyung.  Are you suggesting checking instead for a hardcoded veneer
> symbol string?
>

Veneers don't have symbol strings. Veneers are anonymous sequences of
instructions living in a patch of R-X mapped module space somewhere.
The only symbol strings are for the routines that generate these
veneers, not for the veneers themselves.


>> > +                               pr_debug("NO ERR FOR SKEW %ld: %#" PRIx64 ": diff start addr v: %s k: %#" PRIx64 " %s\n",
>> > +                                        skew, mem_start, sym->name, UM(pair->start), pair->name);
>> > +                               continue;
>> > +                       }
>> > +
>> > +                       pr_debug("ERR : %#" PRIx64 ": diff start addr v: %s k: %#" PRIx64 " %s\n",
>> > +                                mem_start, sym->name, UM(pair->start), pair->name);
>> > +
>> > +                       if (kmap && vmap) {
>> > +                               pr_debug("    : map v: %s k: %s\n",
>> > +                                        vmap->dso->short_name, kmap->dso->short_name);
>> > +                       }
>> >                 } else
>> >                         pr_debug("ERR : %#" PRIx64 ": %s not on kallsyms\n",
>> >                                  mem_start, sym->name);