Re: [RFC PATCH bpf-next v1 01/11] bpf: Give a helper a way to call its callback with a pad

[Kumar Kartikeya Dwivedi <memxor@gmail.com>]

On Wed Oct 28, 2026 at 2:20 AM CET, sashiko-bot wrote:
> [...]
> [Severity: High]
> Can evaluating the macro arguments in ___bpf_call_cb() on arm64 and
> __bpf_call_cb() on x86-64 clobber the caller-saved local register
> variables before the asm_goto_output() statement is reached?

Right: the compiler keeps no local register variable across a call, and
a call in the evaluation of a later argument clobbers the ones set
before it. The next version evaluates every argument into a temporary
first and sets the register variables from those.

> [...]
> [Severity: High]
> Could the compiler eliminate the *slot = to store in bpf_unwind_leave()
> (on both x86-64 and arm64) as a dead store to the current function's stack
> frame right before returning?

Right, and gcc 15 does drop it, on arm64 and x86-64 alike: that is why
the gcc runs of BPF CI returned from every helper but the array iterator
instead of unwinding. The next version stores with WRITE_ONCE(), on the
shadow call stack too.

> [...]