On Tue, Jul 2, 2024 at 4:39 PM Josh Poimboeuf <jpoimboe@xxxxxxxxxx> wrote: > > On Tue, Jul 02, 2024 at 04:35:56PM -0700, Josh Poimboeuf wrote: > > On Tue, Jul 02, 2024 at 10:18:58AM -0700, Andrii Nakryiko wrote: > > > When tracing user functions with uprobe functionality, it's common to > > > install the probe (e.g., a BPF program) at the first instruction of the > > > function. This is often going to be `push %rbp` instruction in function > > > preamble, which means that within that function frame pointer hasn't > > > been established yet. This leads to consistently missing an actual > > > caller of the traced function, because perf_callchain_user() only > > > records current IP (capturing traced function) and then following frame > > > pointer chain (which would be caller's frame, containing the address of > > > caller's caller). > > > > > > So when we have target_1 -> target_2 -> target_3 call chain and we are > > > tracing an entry to target_3, captured stack trace will report > > > target_1 -> target_3 call chain, which is wrong and confusing. > > > > > > This patch proposes a x86-64-specific heuristic to detect `push %rbp` > > > (`push %ebp` on 32-bit architecture) instruction being traced. Given > > > entire kernel implementation of user space stack trace capturing works > > > under assumption that user space code was compiled with frame pointer > > > register (%rbp/%ebp) preservation, it seems pretty reasonable to use > > > this instruction as a strong indicator that this is the entry to the > > > function. In that case, return address is still pointed to by %rsp/%esp, > > > so we fetch it and add to stack trace before proceeding to unwind the > > > rest using frame pointer-based logic. > > > > > > Signed-off-by: Andrii Nakryiko <andrii@xxxxxxxxxx> > > > > Should it also check for ENDBR64? > > Sure, I can add a check for endbr64 as well. endbr64 probably can be used not just at function entry, is that right? So it might be another case of false positive (which I think is ok, see below). > > When compiled with -fcf-protection=branch, the first instruction of the > > function will almost always be ENDBR64. I'm not sure about other > > distros, but at least Fedora compiles its binaries like that. > > BTW, there are some cases (including leaf functions and some stack > alignment sequences) where a "push %rbp" can happen inside a function. > Then it would presumably add a bogus trace entry. Are such false > positives ok? I think such cases should be rare. People mostly seem to trace user function entry/exit, rarely if ever they trace something within the function, except for USDT cases, where it will be a nop instruction that they trace. In general, even with false positives, I think it's overwhelmingly better to get correct entry stack trace 99.9% of the time, and in the rest 0.01% cases it's fine having one extra bogus entry (but the rest should still be correct), which should be easy for humans to recognize and filter out, if necessary. > > -- > Josh
