From: Andrew Waterman <andrew@sifive.com>
To: Palmer Dabbelt <palmer@rivosinc.com>
Cc: libc-alpha@sourceware.org, Vineet Gupta <vineetg@rivosinc.com>
Subject: Re: [PATCH] RISC-V: Clobber V state on system calls
Date: Wed, 27 Mar 2024 14:48:45 -0700 [thread overview]
Message-ID: <CA++6G0CX19CSS7tVtgzKwUwf20GtGmzTHs9t2aEfam-bv3QkGw@mail.gmail.com> (raw)
In-Reply-To: <20240327193601.28903-2-palmer@rivosinc.com>
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
>
next prev parent reply other threads:[~2024-03-27 21:49 UTC|newest]
Thread overview: 4+ messages / expand[flat|nested] mbox.gz Atom feed top
2024-03-27 19:36 [PATCH] RISC-V: Clobber V state on system calls Palmer Dabbelt
2024-03-27 21:48 ` Andrew Waterman [this message]
2024-03-27 21:53 ` Palmer Dabbelt
2024-03-27 22:16 ` Vineet Gupta
Reply instructions:
You may reply publicly to this message via plain-text email
using any one of the following methods:
* Save the following mbox file, import it into your mail client,
and reply-to-all from there: mbox
Avoid top-posting and favor interleaved quoting:
https://en.wikipedia.org/wiki/Posting_style#Interleaved_style
List information: https://www.gnu.org/software/libc/involved.html
* Reply using the --to, --cc, and --in-reply-to
switches of git-send-email(1):
git send-email \
--in-reply-to=CA++6G0CX19CSS7tVtgzKwUwf20GtGmzTHs9t2aEfam-bv3QkGw@mail.gmail.com \
--to=andrew@sifive.com \
--cc=libc-alpha@sourceware.org \
--cc=palmer@rivosinc.com \
--cc=vineetg@rivosinc.com \
/path/to/YOUR_REPLY
https://kernel.org/pub/software/scm/git/docs/git-send-email.html
* If your mail client supports setting the In-Reply-To header
via mailto: links, try the mailto: link
Be sure your reply has a Subject: header at the top and a blank line
before the message body.
This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox;
as well as URLs for read-only IMAP folder(s) and NNTP newsgroup(s).