linux-kernel - Re: perf-probe crash in dwarf_getcfi

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Date:	Sun, 4 Jan 2015 00:07:43 +0900
From:	Namhyung Kim <namhyung@...nel.org>
To:	Mark Wielaard <mjw@...hat.com>
Cc:	David Ahern <dsahern@...il.com>,
	Masami Hiramatsu <masami.hiramatsu.pt@...achi.com>,
	Arnaldo Carvalho de Melo <acme@...nel.org>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: perf-probe crash in dwarf_getcfi_elf

Hi Mark,

On Thu, Jan 1, 2015 at 4:42 AM, Mark Wielaard <mjw@...hat.com> wrote:
> On Tue, Dec 30, 2014 at 06:05:33PM +0900, Namhyung Kim wrote:
>> And I found a related commit (5704c8c4fa71 "getcfi_scn_eh_frame: Don't
>> crash and burn when .eh_frame bits aren't there.") in elfutils that
>> can lead to a unexpected crash like this.  To safely use the function,
>> it needs to check the .eh_frame section is a PROGBITS type.
>
> Sorry about that bug. It was fixed in elfutils 0.156, released July 2013.
> Latest elfutils release is 0.161.
>
> Checking .eh_frame is SHT_PROGBITS (and not SHT_NOBITS) looks like the
> correct workaround if you have to deal with older elfutils releases.

Thanks for confirming this.

>
> Note that dwarf_getcfi_elf gets the CFI information from the .eh_frame
> section associated with the ELF file underlying the Dwarf dbg and needs
> to be released with a call to dwarf_cfi_end (). dwarf_getcfi () gets the
> CFI information from the .debug_frame section data of the Dwarf dbg
> and gets automatically releassed when dwarf_end () is called (you should
> not call dwarf_cfi_end () on the result).

Ah, it's good to know.  I have to check the code..

>
> If the Dwarf dbg handle is associated with a separate debuginfo file,
> then the .eh_frame section data will not be available (has SHT_NOBITS)
> and you can get the .eh_frame section (and call dwarf_getcfi_elf ()) on
> the original ELF file from which the Dwarf debuginfo file was split off.
>
> In your patch you call dwarf_getcfi () as backup to get some CFI
> information when you cannot call dwarf_getcfi_elf (). In general it
> might make sense to also do this when dwarf_getcfi_elf fails. Or try
> to get the CFI through both .eh_frame (from the executable ELF file)
> with dwarf_getcfi_elf () and .debug_frame (from the Dwarf dbg file)
> with dwarf_getcfi () and try to match an address in both tables
> if one or the other fails.

Thanks for your advice, I'll make a follow-up patch to address your comments.

Thanks,
Namhyung
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