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? 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. -- Josh
