git: fa3d164dd9ef - stable/13 - linux(4): Preserve %rcx (return address) like a Linux do

The branch stable/13 has been updated by dchagin:

URL: https://cgit.FreeBSD.org/src/commit/?id=fa3d164dd9ef6fadac6ed4f747ffb09657c5ae76

commit fa3d164dd9ef6fadac6ed4f747ffb09657c5ae76
Author:     Dmitry Chagin <dchagin@FreeBSD.org>
AuthorDate: 2023-05-28 14:06:47 +0000
Commit:     Dmitry Chagin <dchagin@FreeBSD.org>
CommitDate: 2023-06-29 08:16:02 +0000

    linux(4): Preserve %rcx (return address) like a Linux do
    
    Perhaps, this does not makes much sense as destroyng %rcx declared by
    the x86_64 Linux syscall ABI. However,:
    a) if we get a signal while we are in the kernel, we should restore
       tf_rcx when preparing machine context for signal handlers.
    b) the Linux world is strange, someone can depend on %rcx value
       after syscall, something like go.
    
    Differential Revision:  https://reviews.freebsd.org/D40155
    MFC after:              1 month
    
    (cherry picked from commit 037b60fb0fbcb2dd6b6cbb632b05a2939ff4ff29)
---
 sys/amd64/linux/linux_sysvec.c | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/sys/amd64/linux/linux_sysvec.c b/sys/amd64/linux/linux_sysvec.c
index fea1e274ac95..53eda1cbc7c0 100644
--- a/sys/amd64/linux/linux_sysvec.c
+++ b/sys/amd64/linux/linux_sysvec.c
@@ -168,6 +168,8 @@ linux_fetch_syscall_args(struct thread *td)
 
 	/* Restore r10 earlier to avoid doing this multiply times. */
 	frame->tf_r10 = frame->tf_rcx;
+	/* Restore %rcx for machine context. */
+	frame->tf_rcx = frame->tf_rip;
 
 	td->td_retval[0] = 0;
 	return (0);