Re: [musl] add loongarch64 port v3

[Rich Felker <dalias@libc.org>]

On Wed, May 25, 2022 at 06:08:23PM +0800, 王洪亮 wrote:
> 
> 在 2022/5/24 下午8:32, Rich Felker 写道:
> >What we've been trying to say is that there are several cases, none of
> >which seem to need it:
> >
> >1. You create an object with declared type struct sigcontext. In this
> >    case, the flexible array member at the end is not present at all
> >    (because that's how C works) which means there's no extended
> >    context which needs additional alignment and probably also means
> >    this is not a usable way of creating sigcontext structs.
> >
> >2. You malloc storage for the object with space for the flexible array
> >    member. In this case the allocation has alignment max_align_t and
> >    everything is fine.
> >
> >3. You get the object from the kernel pushing it onto the stack in a
> >    signal frame. This is probably actually the only case the type is
> >    usable in, and of course it has whatever alignment the kernel gave
> >    it.
> Specify the __attribute__((__aligned__(16))) in musl,is used to be
> consistent with kernel.if I removed the __attribute__((__aligned__(16))),
> there is a libc-test fail in pthread_cancel.exe.the reason is that the
> offset of uc->uc_mcontext from the start of uc obtained in cancel_handler
> is inconsistent with kernel pushing it onto the stack in a signal frame.
> so I understand the __attribute__((__aligned__(16))) is necessary in musl.
> 
> src/thread/pthread_cancel.c
> 
> static void cancel_handler(int sig, siginfo_t *si, void *ctx)
> {
>         pthread_t self = __pthread_self();
>         ucontext_t *uc = ctx;
>         uintptr_t pc = uc->uc_mcontext.MC_PC;
>          ...
> }
> 
> musl/arch/loongarch64/bits/signal.h:
> 
> typedef unsigned long greg_t, gregset_t[32];
> typedef struct sigcontext {
>         unsigned long pc;
>         gregset_t gregs;
>         unsigned int flags;
>         unsigned long extcontext[];
> }mcontext_t;
> 
> linux/arch/loongarch/include/uapi/asm/sigcontext.h:
> 
> struct sigcontext {
>         __u64   sc_pc;
>         __u64   sc_regs[32];
>         __u32   sc_flags;
>         __u64   sc_extcontext[0] __attribute__((__aligned__(16)));
> };

This is because ucontext_t is defined without explicit padding before
uc_mcontext. Add "long __uc_pad;" or similar before it so that the
offset is explicitly what it's supposed to be rather than a
consequence ot overalignment.

Rich