Re: [PATCH] x86/static_call: Fix __static_call_fixup()

Steven Rostedt
2023-08-16T00:42:19Z

On Wed, 16 Aug 2023 01:08:09 +0200
Peter Zijlstra <peterz@infradead.org> wrote:

> diff --git a/arch/x86/kernel/static_call.c b/arch/x86/kernel/static_call.c
> index b70670a98597..2e67512d7104 100644
> --- a/arch/x86/kernel/static_call.c
> +++ b/arch/x86/kernel/static_call.c
> @@ -186,6 +186,16 @@ EXPORT_SYMBOL_GPL(arch_static_call_transform);
> */
> bool __static_call_fixup(void *tramp, u8 op, void *dest)
> {
> + /*
> + * Not all .return_sites are a static_call trampoline (most are not).
> + * Check if the next 3 bytes are still kernel text, if not, then this
> + * definitely is not a trampoline and we need not worry further.
> + *
> + * This avoids the memcmp() below tripping over pagefaults etc..
> + */
> + if (!kernel_text_address(tramp+7))

The comment says "next 3 bytes" and the test is "tramp+7". Why the magic 7 number?

If the tramp is 5 bytes, shouldn't it be +8?

-- Steve

> + return false;
> +
> if (memcmp(tramp+5, tramp_ud, 3)) {
> /* Not a trampoline site, not our problem. */
> return false;