Re: [musl] [PATCH] arm64/sigcontext: Synchronize the type of the __reserved field with the linux kernel.

[Rich Felker <dalias@libc.org>]

On Thu, Aug 19, 2021 at 07:54:12AM +0200, Florian Weimer wrote:
> * Rich Felker:
> 
> > On Thu, Aug 19, 2021 at 12:52:23AM +0200, Olivier Galibert wrote:
> >> clang's compiler-rt sanitizer_linux.cpp expects the __reserved field
> >> to be convertible to u8 *.  So let's.
> >> ---
> >>  arch/aarch64/bits/signal.h | 2 +-
> >>  1 file changed, 1 insertion(+), 1 deletion(-)
> >> 
> >> diff --git a/arch/aarch64/bits/signal.h b/arch/aarch64/bits/signal.h
> >> index 5098c734..a46997e3 100644
> >> --- a/arch/aarch64/bits/signal.h
> >> +++ b/arch/aarch64/bits/signal.h
> >> @@ -19,7 +19,7 @@ typedef struct sigcontext {
> >>  	unsigned long fault_address;
> >>  	unsigned long regs[31];
> >>  	unsigned long sp, pc, pstate;
> >> -	long double __reserved[256];
> >> +	unsigned char __reserved[4096] __attribute__((__aligned__(16)));
> >>  } mcontext_t;
> >>  
> >>  #define FPSIMD_MAGIC 0x46508001
> >
> > The member name __reserved is not API, much less its particular type.
> 
> The name is called __reserved, but it is actually part of the API.
> We learned this when we tried to rename it:
> 
>   <https://sourceware.org/bugzilla/show_bug.cgi?id=22742>
> 
> The name and its __ prefix are rather unfortunate, but we are stuck with
> it.

I question the reasoning there. Just because there are users of it
doesn't mean it's API, *especially* if the users are things like
sanitizer lib that regularly poke at internals that are not interface
contracts. Use of a name like __reserved as API is *really* bad since
it could even be something like a macro for an attribute in the
implementation, rather than something available as a member name.

My interpretation was that it's something like the powerpc reserved
space where there's a separate pointer into it, which you're supposed
to access it by. But that doesn't seem to be the case here, so I'm not
sure what the right way to access it is. Do you have a list of
software that's actually poking at it so we can evaluate the situation
better for figuring out what to do?

Rich