FAILED: patch "[PATCH] riscv: entry: always initialize regs->a0 to -ENOSYS" failed to apply to 6.6-stable tree

[<gregkh@linuxfoundation.org>]

The patch below does not apply to the 6.6-stable tree.
If someone wants it applied there, or to any other stable or longterm
tree, then please email the backport, including the original git commit
id to <stable@vger.kernel.org>.

To reproduce the conflict and resubmit, you may use the following commands:

git fetch https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/ linux-6.6.y
git checkout FETCH_HEAD
git cherry-pick -x 61119394631f219e23ce98bcc3eb993a64a8ea64
# <resolve conflicts, build, test, etc.>
git commit -s
git send-email --to '<stable@vger.kernel.org>' --in-reply-to '2024081917-flanked-clear-e564@gregkh' --subject-prefix 'PATCH 6.6.y' HEAD^..

Possible dependencies:

61119394631f ("riscv: entry: always initialize regs->a0 to -ENOSYS")
05d450aabd73 ("riscv: Support RANDOMIZE_KSTACK_OFFSET")

thanks,

greg k-h

------------------ original commit in Linus's tree ------------------

From 61119394631f219e23ce98bcc3eb993a64a8ea64 Mon Sep 17 00:00:00 2001
From: Celeste Liu <coelacanthushex@gmail.com>
Date: Thu, 27 Jun 2024 22:23:39 +0800
Subject: [PATCH] riscv: entry: always initialize regs->a0 to -ENOSYS
MIME-Version: 1.0
Content-Type: text/plain; charset=UTF-8
Content-Transfer-Encoding: 8bit

Otherwise when the tracer changes syscall number to -1, the kernel fails
to initialize a0 with -ENOSYS and subsequently fails to return the error
code of the failed syscall to userspace. For example, it will break
strace syscall tampering.

Fixes: 52449c17bdd1 ("riscv: entry: set a0 = -ENOSYS only when syscall != -1")
Reported-by: "Dmitry V. Levin" <ldv@strace.io>
Reviewed-by: Björn Töpel <bjorn@rivosinc.com>
Cc: stable@vger.kernel.org
Signed-off-by: Celeste Liu <CoelacanthusHex@gmail.com>
Link: https://lore.kernel.org/r/20240627142338.5114-2-CoelacanthusHex@gmail.com
Signed-off-by: Palmer Dabbelt <palmer@rivosinc.com>

diff --git a/arch/riscv/kernel/traps.c b/arch/riscv/kernel/traps.c
index 05a16b1f0aee..51ebfd23e007 100644
--- a/arch/riscv/kernel/traps.c
+++ b/arch/riscv/kernel/traps.c
@@ -319,6 +319,7 @@ void do_trap_ecall_u(struct pt_regs *regs)
 
 		regs->epc += 4;
 		regs->orig_a0 = regs->a0;
+		regs->a0 = -ENOSYS;
 
 		riscv_v_vstate_discard(regs);
 
@@ -328,8 +329,7 @@ void do_trap_ecall_u(struct pt_regs *regs)
 
 		if (syscall >= 0 && syscall < NR_syscalls)
 			syscall_handler(regs, syscall);
-		else if (syscall != -1)
-			regs->a0 = -ENOSYS;
+
 		/*
 		 * Ultimately, this value will get limited by KSTACK_OFFSET_MAX(),
 		 * so the maximum stack offset is 1k bytes (10 bits).