Messages in this thread | | | From | Mathieu Poirier <> | Date | Thu, 15 Aug 2019 14:16:48 -0600 | Subject | Re: [PATCH v3 3/4] perf: Use CAP_SYSLOG with kptr_restrict checks |
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On Wed, 14 Aug 2019 at 14:02, Lubashev, Igor <ilubashe@akamai.com> wrote: > > > On Wed, August 14, 2019 at 2:52 PM Arnaldo Carvalho de Melo <arnaldo.melo@gmail.com> wrote: > > Em Wed, Aug 14, 2019 at 03:48:14PM -0300, Arnaldo Carvalho de Melo > > escreveu: > > > Em Wed, Aug 14, 2019 at 12:04:33PM -0600, Mathieu Poirier escreveu: > > > > # echo 0 > /proc/sys/kernel/kptr_restrict # ./tools/perf/perf record > > > > -e instructions:k uname > > > > perf: Segmentation fault > > > > Obtained 10 stack frames. > > > > ./tools/perf/perf(sighandler_dump_stack+0x44) [0x55af9e5da5d4] > > > > /lib/x86_64-linux-gnu/libc.so.6(+0x3ef20) [0x7fd31efb6f20] > > > > ./tools/perf/perf(perf_event__synthesize_kernel_mmap+0xa7) > > > > [0x55af9e590337] > > > > ./tools/perf/perf(+0x1cf5be) [0x55af9e50c5be] > > > > ./tools/perf/perf(cmd_record+0x1022) [0x55af9e50dff2] > > > > ./tools/perf/perf(+0x23f98d) [0x55af9e57c98d] > > > > ./tools/perf/perf(+0x23fc9e) [0x55af9e57cc9e] > > > > ./tools/perf/perf(main+0x369) [0x55af9e4f6bc9] > > > > /lib/x86_64-linux-gnu/libc.so.6(__libc_start_main+0xe7) > > > > [0x7fd31ef99b97] > > > > ./tools/perf/perf(_start+0x2a) [0x55af9e4f704a] Segmentation fault > > > > > > > > I can reproduce this on both x86 and ARM64. > > > > > > I don't see this with these two csets removed: > > > > > > 7ff5b5911144 perf symbols: Use CAP_SYSLOG with kptr_restrict checks > > > d7604b66102e perf tools: Use CAP_SYS_ADMIN with perf_event_paranoid > > > checks > > > > > > Which were the ones I guessed were related to the problem you > > > reported, so they are out of my ongoing perf/core pull request to > > > Ingo/Thomas, now trying with these applied and your instructions... > > > > Can't repro: > > > > [root@quaco ~]# cat /proc/sys/kernel/kptr_restrict > > 0 > > [root@quaco ~]# perf record -e instructions:k uname Linux [ perf record: > > Woken up 1 times to write data ] [ perf record: Captured and wrote 0.024 MB > > perf.data (1 samples) ] [root@quaco ~]# echo 1 > > > /proc/sys/kernel/kptr_restrict [root@quaco ~]# perf record -e instructions:k > > uname Linux [ perf record: Woken up 1 times to write data ] [ perf record: > > Captured and wrote 0.024 MB perf.data (1 samples) ] [root@quaco ~]# echo > > 0 > /proc/sys/kernel/kptr_restrict [root@quaco ~]# perf record -e > > instructions:k uname Linux [ perf record: Woken up 1 times to write data ] [ > > perf record: Captured and wrote 0.024 MB perf.data (1 samples) ] > > [root@quaco ~]# > > > > [acme@quaco perf]$ git log --oneline --author Lubashev tools/ > > 7ff5b5911144 (HEAD -> perf/cap, acme.korg/tmp.perf/cap, > > acme.korg/perf/cap) perf symbols: Use CAP_SYSLOG with kptr_restrict > > checks d7604b66102e perf tools: Use CAP_SYS_ADMIN with > > perf_event_paranoid checks c766f3df635d perf ftrace: Use CAP_SYS_ADMIN > > instead of euid==0 c22e150e3afa perf tools: Add helpers to use capabilities if > > present > > 74d5f3d06f70 tools build: Add capability-related feature detection perf > > version 5.3.rc4.g7ff5b5911144 [acme@quaco perf]$ > > I got an ARM64 cloud VM, but I cannot reproduce. > # cat /proc/sys/kernel/kptr_restrict > 0 > > Perf trace works fine (does not die): > # ./perf trace -a > > Here is my setup: > Repo: git://git.kernel.org/pub/scm/linux/kernel/git/acme/linux.git > Branch: tmp.perf/cap > Commit: 7ff5b5911 "perf symbols: Use CAP_SYSLOG with kptr_restrict checks" > gcc --version: gcc (Ubuntu/Linaro 7.4.0-1ubuntu1~18.04.1) 7.4.0 > uname -a: Linux arm-4-par-1 4.9.93-mainline-rev1 #1 SMP Tue Apr 10 09:54:46 UTC 2018 aarch64 aarch64 aarch64 GNU/Linux > lsb_release -a: Ubuntu 18.04.3 LTS > > Auto-detecting system features: > ... dwarf: [ on ] > ... dwarf_getlocations: [ on ] > ... glibc: [ on ] > ... gtk2: [ on ] > ... libaudit: [ on ] > ... libbfd: [ on ] > ... libcap: [ on ] > ... libelf: [ on ] > ... libnuma: [ on ] > ... numa_num_possible_cpus: [ on ] > ... libperl: [ on ] > ... libpython: [ on ] > ... libcrypto: [ on ] > ... libunwind: [ on ] > ... libdw-dwarf-unwind: [ on ] > ... zlib: [ on ] > ... lzma: [ on ] > ... get_cpuid: [ OFF ] > ... bpf: [ on ] > ... libaio: [ on ] > ... libzstd: [ on ] > ... disassembler-four-args: [ on ] > > I also could not reproduce on x86: > lsb_release -a: Ubuntu 18.04.1 LTS > gcc --version: gcc (Ubuntu 7.4.0-1ubuntu1~18.04aka10.0.0) 7.4.0 > uname -r: 4.4.0-154-generic
I isolated the problem to libcap-dev - if it is not installed a segmentation fault will occur. Since this set is about using capabilities it is obvious that not having it on a system should fail a trace session, but it should not crash it.
If libcap-dev is not installed function symbol__restricted_filename() will return true, which in turn will prevent symbol_name to be set in machine__get_running_kernel_start(). That prevents function map__set_kallsyms_ref_reloc_sym() from being called in machine__create_kernel_maps(), resulting in kmap->ref_reloc_sym being NULL in _perf_event__synthesize_kernel_mmap() and a segmentation fault.
I am not sure how this can be fixed. I counted a total of 19 instances where kmap->ref_reloc_sym->XYZ is called, only 2 of wich care to check if kmap->ref_reloc_sym is valid before proceeding. As such I must hope that in the 17 other cases, kmap->ref_reloc_sym is guaranteed to be valid. If I am correct then all we need is to check for a valid pointer in _perf_event__synthesize_kernel_mmap(). Otherwise it will be a little harder.
Mathieu
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