Re: [PATCH v2] perf,x86: avoid missing caller address in stack traces captured in uprobe

[Josh Poimboeuf <jpoimboe@kernel.org>]

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@kernel.org>
> 
> 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.

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?

-- 
Josh