[Bug target/106671] aarch64: BTI instruction are not inserted for cross-section direct calls

["wilco at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=106671

Wilco <wilco at gcc dot gnu.org> changed:

           What    |Removed                     |Added
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                 CC|                            |wilco at gcc dot gnu.org

--- Comment #10 from Wilco <wilco at gcc dot gnu.org> ---
(In reply to Feng Xue from comment #9)
> On some occasions, we may not use the new ld, the kernel-building relies on
> its own runtime linker which is used for kernel modules. So I created a
> patch (https://gcc.gnu.org/pipermail/gcc-patches/2023-August/626084.html),
> and this provides user another option that could be done at the compiler
> side.

Reducing BTI is important for security. With LTO a binary should only have BTI
on functions that are indirectly called. So I don't like the idea of adding
more BTI with a new option - it means we will need a linker optimization to
remove those redundant BTIs (eg. by changing them into NOPs).

Note that branch offsets up to 256MB don't need special veneer handling: one
should place a direct branch about halfway to the destination.

Does Linux do any weird hacks in -fpatchable-function-entry that makes it hard
to use BTI?