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 -- Josh
