Re: 'perf probe' and symbols from .text.<something>
From: Masami Hiramatsu
Date: Wed Feb 24 2021 - 03:03:08 EST
On 2021年2月23日 13:45:46 -0600
Josh Poimboeuf <jpoimboe@xxxxxxxxxx> wrote:
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On Tue, Feb 23, 2021 at 04:36:19PM +0900, Masami Hiramatsu wrote:
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> On 2021年2月23日 10:23:31 +0900
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> Masami Hiramatsu <mhiramat@xxxxxxxxxx> wrote:
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>
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> > On 2021年2月22日 11:51:50 -0600
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> > Josh Poimboeuf <jpoimboe@xxxxxxxxxx> wrote:
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> >
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> > > On Tue, Feb 23, 2021 at 12:05:08AM +0900, Masami Hiramatsu wrote:
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> > > > > Of course, one could place probes using absolute addresses of the
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> > > > > functions but that would be less convenient.
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> > > > >
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> > > > > This also affects many livepatch modules where the kernel code can be
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> > > > > compiled with -ffunction-sections and each function may end up in a
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> > > > > separate section .text.<function_name>. 'perf probe' cannot be used
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> > > > > there, except with the absolute addresses.
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> > > > >
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> > > > > Moreover, if FGKASLR patches are merged
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> > > > > (https://lwn.net/Articles/832434/) and the kernel is built with FGKASLR
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> > > > > enabled, -ffunction-sections will be used too. 'perf probe' will be
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> > > > > unable to see the kernel functions then.
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> > > >
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> > > > Hmm, if the FGKASLAR really randomizes the symbol address, perf-probe
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> > > > should give up "_text-relative" probe for that kernel, and must fallback
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> > > > to the "symbol-based" probe. (Are there any way to check the FGKASLR is on?)
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> > > > The problem of "symbol-based" probe is that local (static) symbols
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> > > > may share a same name sometimes. In that case, it can not find correct
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> > > > symbol. (Maybe I can find a candidate from its size.)
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> > > > Anyway, sometimes the security and usability are trade-off.
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> > >
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> > > We had a similar issue with FGKASLR and live patching. The proposed
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> > > solution is a new linker flag which eliminates duplicates: -z
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> > > unique-symbol.
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> > >
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> > > https://sourceware.org/bugzilla/show_bug.cgi?id=26391
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> >
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> > Interesting, but it might not be enough for perf-probe.
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> > Since the perf-probe has to handle both dwarf and elf, both must be
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> > changed. I think the problem is that the dwarf is generated while
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> > compiling, but this -z seems converting elf symbols in linkage.
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> > As far as I can see, this appends ".COUNT" suffix to the non-unique
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> > symbols in the linkage phase. Is that also applied to dwarf too?
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>
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> Ah, OK. If there is an offline elf binary with symbol map, I can convert
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> DWARF symbol -> address -> offline elf symbol (unique name)-> kallsyms.
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> Currently, it directly converts address by kallsyms, so I will change it
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> to find elf-symbol and solve address by kallsyms in post processing.
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>
DWARF sections have references to the ELF symbols, which are renamed by
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the linker. So DWARF should automatically show the new symbol name.
OK, I'll check what elfutils provides about that information.
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And kallsyms is generated after the kernel is linked. So I'm not sure I
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understand the problem.
Actually, perf-probe currently uses subprogram DIE(Dwarf node) name for
the symbol name and post-process tries to find correct symbol name
from kallsyms by the address.
So I have to change it to find the ELF symbol name from DIE itself.
Thank you,
--
Masami Hiramatsu <mhiramat@xxxxxxxxxx>