[PATCH v4 4/4] arm64: implement per-function metadata storage for arm64

[Sami Tolvanen]

On Mon, Mar 03, 2025 at 09:28:37PM +0800, Menglong Dong wrote:
> The per-function metadata storage is already used by ftrace if
> CONFIG_DYNAMIC_FTRACE_WITH_CALL_OPS is enabled, and it store the pointer
> of the callback directly to the function padding, which consume 8-bytes,
> in the commit
> baaf553d3bc3 ("arm64: Implement HAVE_DYNAMIC_FTRACE_WITH_CALL_OPS").
> So we can directly store the index to the function padding too, without
> a prepending. With CONFIG_DYNAMIC_FTRACE_WITH_CALL_OPS enabled, the
> function is 8-bytes aligned, and we will compile the kernel with extra
> 8-bytes (2 NOPS) padding space. Otherwise, the function is 4-bytes
> aligned, and only extra 4-bytes (1 NOPS) is needed.
> 
> However, we have the same problem with Mark in the commit above: we can't
> use the function padding together with CFI_CLANG, which can make the clang
> compiles a wrong offset to the pre-function type hash. He said that he was
> working with others on this problem 2 years ago. Hi Mark, is there any
> progress on this problem?

I don't think there's been much progress since the previous
discussion a couple of years ago. The conclusion seemed to be
that adding a section parameter to -fpatchable-function-entry
would allow us to identify notrace functions while keeping a
consistent layout for functions:

https://lore.kernel.org/lkml/Y1QEzk%2FA41PKLEPe@hirez.programming.kicks-ass.net/

Sami