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

[Menglong Dong]

On Tue, Mar 4, 2025 at 5:40 AM Sami Tolvanen <samitolvanen@xxxxxxxxxx> wrote:
>
> 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@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx/

Thank you for your information, which helps me a lot.
I'll dig deeper to find a way to keep CFI working together
with this function.

Thanks!
Menglong Dong

>
> Sami