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