Re: git: d957343f8713 - main - linux(4): Rework signal trampoline on Aarch64

On 14 May 2023, at 22:31, Dmitry Chagin <dchagin@FreeBSD.org> wrote:
> 
> The branch main has been updated by dchagin:
> 
> URL: https://cgit.FreeBSD.org/src/commit/?id=d957343f8713a312442a0140b7a7752ed7b2d870
> 
> commit d957343f8713a312442a0140b7a7752ed7b2d870
> Author:     Dmitry Chagin <dchagin@FreeBSD.org>
> AuthorDate: 2023-05-14 21:27:31 +0000
> Commit:     Dmitry Chagin <dchagin@FreeBSD.org>
> CommitDate: 2023-05-14 21:27:31 +0000
> 
>    linux(4): Rework signal trampoline on Aarch64
> 
>    To avoid clobbering of any registers by the trampoline code use Linux
>    way to call signal handlers. I.e., we are out from the kernel right into
>    the signal handler, put return address from the signal handler into the
>    link register.
>    The mysterious NOP is required for some unwinders (e.g. libc++) that
>    unconditionally subtract one from the result of _Unwind_GetIP() in order
>    to identify the calling function.
> 
>    MFC after:              1 week

So this is basically a revert of c56480a83235 "linux(4): Implement signal
trampoline for arm64 in a FreeBSD-way”? Not a huge fan of this flip-flopping
without much justification and with no review.

Jess

> ---
> sys/arm64/linux/linux_locore.asm | 5 +++--
> sys/arm64/linux/linux_sysvec.c   | 6 +++---
> sys/arm64/linux/linux_vdso.lds.s | 1 +
> 3 files changed, 7 insertions(+), 5 deletions(-)
> 
> diff --git a/sys/arm64/linux/linux_locore.asm b/sys/arm64/linux/linux_locore.asm
> index 6ebecef51b39..de3e5dd52a00 100644
> --- a/sys/arm64/linux/linux_locore.asm
> +++ b/sys/arm64/linux/linux_locore.asm
> @@ -45,10 +45,11 @@ linux_platform:
> 
> .text
> 
> - nop /* This is what Linux calls a "Mysterious NOP". */
> EENTRY(__kernel_rt_sigreturn)
> - blr x8
> + nop /* This is what Linux calls a "Mysterious NOP". */
> 
> + .globl __user_rt_sigreturn
> +__user_rt_sigreturn:
> mov x8, #LINUX_SYS_linux_rt_sigreturn
> svc #0
> EEND(__kernel_rt_sigreturn)
> diff --git a/sys/arm64/linux/linux_sysvec.c b/sys/arm64/linux/linux_sysvec.c
> index 299586e1c7b6..bb9ff25893eb 100644
> --- a/sys/arm64/linux/linux_sysvec.c
> +++ b/sys/arm64/linux/linux_sysvec.c
> @@ -118,7 +118,7 @@ LIN_SDT_PROBE_DEFINE0(sysvec, linux_exec_setregs, todo);
> 
> LINUX_VDSO_SYM_CHAR(linux_platform);
> LINUX_VDSO_SYM_INTPTR(kern_timekeep_base);
> -LINUX_VDSO_SYM_INTPTR(__kernel_rt_sigreturn);
> +LINUX_VDSO_SYM_INTPTR(__user_rt_sigreturn);
> 
> static int
> linux_fetch_syscall_args(struct thread *td)
> @@ -353,9 +353,9 @@ linux_rt_sendsig(sig_t catcher, ksiginfo_t *ksi, sigset_t *mask)
> tf->tf_x[2] = 0;
> }
> tf->tf_x[29] = (register_t)&fp->fp;
> - tf->tf_x[8] = (register_t)catcher;
> + tf->tf_elr = (register_t)catcher;
> tf->tf_sp = (register_t)fp;
> - tf->tf_elr = (register_t)__kernel_rt_sigreturn;
> + tf->tf_lr = (register_t)__user_rt_sigreturn;
> 
> CTR3(KTR_SIG, "sendsig: return td=%p pc=%#x sp=%#x", td, tf->tf_elr,
>    tf->tf_sp);
> diff --git a/sys/arm64/linux/linux_vdso.lds.s b/sys/arm64/linux/linux_vdso.lds.s
> index 652b99545069..3f6b70c09176 100644
> --- a/sys/arm64/linux/linux_vdso.lds.s
> +++ b/sys/arm64/linux/linux_vdso.lds.s
> @@ -70,6 +70,7 @@ VERSION
> global:
> linux_platform;
> kern_timekeep_base;
> + __user_rt_sigreturn;
> local: *;
> };
> }


Jess