[PATCH] RISC-V: Clobber V state on system calls
Andrew Waterman
andrew@sifive.com
Wed Mar 27 21:48:45 GMT 2024
LGTM. I suspect this hasn't manifested as a bug because a glibc
routine with an inline syscall would need to be vectorized for this to
be a potential problem. But the prophylaxis is a good idea.
On Wed, Mar 27, 2024 at 2:37 PM Palmer Dabbelt <palmer@rivosinc.com> wrote:
>
> The Linux uABI clobbers all V state on syscalls (similar to SVE), but
> the syscall inline asm macros don't enforce this. So just explicitly
> clobber everything.
>
> Reported-by: Vineet Gupta <vineetg@rivosinc.com>
> Signed-off-by: Palmer Dabbelt <palmer@rivosinc.com>
> ---
> Vineet's been debugging a userspace hang, and it looks like it's
> uncovered at least three issues:
>
> * Linux isn't properly tracking V state, which results in some
> signal-based userpace return paths missing the V state save. This is
> almost certainly a Linux bug, Charlie is looking at it.
> * GCC only discards the V register state on function calls, despite the
> ABI also mandating that the V CSR state is discarded. I'm not 100% on
> this one as I don't really understand the vsetvl passes, but we were
> talking about it on the GCC call yesterday and that's our best guess
> right now.
> * glibc doesn't mark the V state as clobbered by syscalls.
>
> I don't know if we can actually manifest incorrect behavior here and it
> definately doesn't build (GCC doesn't support vxsat [1]). I'm sort of
> just sending this as a placeholder, but I figured with all the other
> chaos I should send it rather than risking forgetting about it.
>
> [1]: https://inbox.sourceware.org/gcc-patches/20240327195403.29732-2-palmer@rivosinc.com/
> ---
> sysdeps/unix/sysv/linux/riscv/sysdep.h | 10 ++++++++++
> 1 file changed, 10 insertions(+)
>
> diff --git a/sysdeps/unix/sysv/linux/riscv/sysdep.h b/sysdeps/unix/sysv/linux/riscv/sysdep.h
> index ee015dfeb6..3e3971e321 100644
> --- a/sysdeps/unix/sysv/linux/riscv/sysdep.h
> +++ b/sysdeps/unix/sysv/linux/riscv/sysdep.h
> @@ -354,7 +354,17 @@
> _sys_result; \
> })
>
> +#ifdef __riscv_vector
> +# define __SYSCALL_CLOBBERS "memory", "vl", "vtype", "vxrm", "vxsat", \
> + "v0", "v1", "v2", "v3", "v4", "v5", \
> + "v6", "v7", "v8", "v9", "v10", "v11", \
> + "v12", "v13", "v14", "v15", "v16", "v17", \
> + "v18", "v18", "v19", "v20", "v21", "v22", \
> + "v23", "v24", "v25", "v26", "v27", "v28", \
> + "v29", "v30", "v31"
> +#else
> # define __SYSCALL_CLOBBERS "memory"
> +#endif
>
> extern long int __syscall_error (long int neg_errno);
>
> --
> 2.44.0
>
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