linux-kernel - [PATCH] riscv: entry: Save a0 prior syscall_enter_from_user

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Message-Id: <20230403065207.1070974-1-bjorn@kernel.org>
Date:   Mon,  3 Apr 2023 08:52:07 +0200
From:   Björn Töpel <bjorn@...nel.org>
To:     Albert Ou <aou@...s.berkeley.edu>,
        Paul Walmsley <paul.walmsley@...ive.com>,
        Palmer Dabbelt <palmer@...belt.com>,
        linux-riscv@...ts.infradead.org, Guo Ren <guoren@...ux.alibaba.com>
Cc:     Björn Töpel <bjorn@...osinc.com>,
        Heiko Stübner <heiko@...ech.de>,
        Conor Dooley <conor@...nel.org>,
        Yipeng Zou <zouyipeng@...wei.com>,
        linux-kernel@...r.kernel.org, Palmer Dabbelt <palmer@...osinc.com>,
        Heiko Stuebner <heiko.stuebner@...ll.eu>
Subject: [PATCH] riscv: entry: Save a0 prior syscall_enter_from_user_mode()

From: Björn Töpel <bjorn@...osinc.com>

The RISC-V calling convention passes the first argument, and the
return value in the a0 register. For this reason, the a0 register
needs some extra care; When handling syscalls, the a0 register is
saved into regs->orig_a0, so a0 can be properly restored for,
e.g. interrupted syscalls.

This functionality was broken with the introduction of the generic
entry patches. Here, a0 was saved into orig_a0 after calling
syscall_enter_from_user_mode(), which can change regs->a0 for some
paths, incorrectly restoring a0.

This is resolved, by saving a0 prior doing the
syscall_enter_from_user_mode() call.

Fixes: f0bddf50586d ("riscv: entry: Convert to generic entry")
Reviewed-by: Heiko Stuebner <heiko.stuebner@...ll.eu>
Tested-by: Heiko Stuebner <heiko.stuebner@...ll.eu>
Signed-off-by: Björn Töpel <bjorn@...osinc.com>
---
 arch/riscv/kernel/traps.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/arch/riscv/kernel/traps.c b/arch/riscv/kernel/traps.c
index 1f4e37be7eb3..8c258b78c925 100644
--- a/arch/riscv/kernel/traps.c
+++ b/arch/riscv/kernel/traps.c
@@ -270,11 +270,11 @@ asmlinkage __visible __trap_section void do_trap_ecall_u(struct pt_regs *regs)
 	if (user_mode(regs)) {
 		ulong syscall = regs->a7;
 
-		syscall = syscall_enter_from_user_mode(regs, syscall);
-
 		regs->epc += 4;
 		regs->orig_a0 = regs->a0;
 
+		syscall = syscall_enter_from_user_mode(regs, syscall);
+
 		if (syscall < NR_syscalls)
 			syscall_handler(regs, syscall);
 		else

base-commit: d34a6b715a23ccd9c9d0bc7a475bea59dc3e28b2
-- 
2.37.2