[PATCH] um: Fix misaligned stack in stub_exe

[Johannes Berg]

On Sat, 2024-10-19 at 13:54 +0800, David Gow wrote:
> 
> Okay, it turns out this breaks clang:
> arch/um/kernel/skas/stub_exe.c:84:2: error: non-ASM statement in naked
> function is not supported

Fun :)

Interesting too, I've _definitely_ got some code elsewhere, that's
usually compiled with clang, using __attribute__((naked,noinline)). With
a quite old clang version, I believe. Oh well, whatever.

> And, looking into it, gcc's docs are not encouraging here either:
> https://gcc.gnu.org/onlinedocs/gcc/x86-Function-Attributes.html
> 
> This attribute allows the compiler to construct the requisite function
> declaration, while allowing the body of the function to be assembly
> code. The specified function will not have prologue/epilogue sequences
> generated by the compiler. Only basic asm statements can safely be
> included in naked functions (see Basic Asm — Assembler Instructions
> Without Operands). While using extended asm or a mixture of basic asm
> and C code may appear to work, they cannot be depended upon to work
> reliably and are not supported.

Right ...

> My gut feeling is that the "correct" way of doing this is to use an
> actual crt implementation for __start. I managed to get it working
> with nolibc on x86_54, but the sheer amount of hackery involved was
> not exactly encouraging. There are a lot of conflicts between the
> different headers for a start.

Yeah, that doesn't seem like a lot of fun.

> The other "correct" way would be to rewrite __start in assembly, which
> is annoying as it'd be architecture specific, so need a separate
> 32-bit version.

That'd probably be less bad than it sounds, since all the syscall magic
macros etc. in there are already architecture specific.

> The less-correct-but-working-here way, which I'm tempted by for now,
> is to get rid of __attribute__((naked)) and just use
> __attribute__((force_arg_align_pointer)). That's probably the best way
> of fixing the stack issue, but obviously won't fix any other issues
> which could arise from __start playing loose with the rules.
> 
> My current plan is to send out a v2 with force_arg_align_pointer next
> week, unless someone has a more brilliant idea.

Sounds good to me. We'll find out if we have to iterate more ;)

johannes