On Mon, Mar 23, 2020 at 12:21:44PM +0000, Catalin Marinas wrote:

On Fri, Mar 20, 2020 at 05:39:46PM +0000, Szabolcs Nagy wrote:

+int arch_elf_adjust_prot(int prot, const struct arch_elf_state *state,
+                        bool has_interp, bool is_interp)
+{
+       if (is_interp != has_interp)
+               return prot;
+
+       if (!(state->flags & ARM64_ELF_BTI))
+               return prot;
+
+       if (prot & PROT_EXEC)
+               prot |= PROT_BTI;
+
+       return prot;
+}

At a quick look, for dynamic binaries we have has_interp == true and
is_interp == false. I don't know why but, either way, the above code
needs a comment with some justification.

I don't really know for certain either, I inherited this code as is with
the understanding that this was all agreed with the toolchain and libc
people - the actual discussion that lead to the decisions being made
happened before I was involved.  My understanding is that the idea was
that the dynamic linker would be responsible for mapping everything in
dynamic applications other than itself but other than consistency I
don't know why.  I guess it defers more decision making to userspace but
I'm having a hard time thinking of sensible cases where one might wish
to make a decision other than enabling PROT_BTI.

I'd be perfectly happy to drop the check if that makes more sense to
people, otherwise I can send a patch adding a comment explaining the
situation.