[PATCH] riscv: entry: Save a0 prior syscall_enter_from_user_mode()

From: "Björn Töpel" <bjorn@kernel.org>
To: Albert Ou <aou@eecs.berkeley.edu>,
	Paul Walmsley <paul.walmsley@sifive.com>,
	Palmer Dabbelt <palmer@dabbelt.com>,
	linux-riscv@lists.infradead.org,
	Guo Ren <guoren@linux.alibaba.com>
Cc: "Björn Töpel" <bjorn@rivosinc.com>,
	"Heiko Stübner" <heiko@sntech.de>,
	"Conor Dooley" <conor@kernel.org>,
	"Yipeng Zou" <zouyipeng@huawei.com>,
	linux-kernel@vger.kernel.org,
	"Palmer Dabbelt" <palmer@rivosinc.com>,
	"Heiko Stuebner" <heiko.stuebner@vrull.eu>
Subject: [PATCH] riscv: entry: Save a0 prior syscall_enter_from_user_mode()
Date: Mon,  3 Apr 2023 08:52:07 +0200	[thread overview]
Message-ID: <20230403065207.1070974-1-bjorn@kernel.org> (raw)

From: Björn Töpel <bjorn@rivosinc.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@vrull.eu>
Tested-by: Heiko Stuebner <heiko.stuebner@vrull.eu>
Signed-off-by: Björn Töpel <bjorn@rivosinc.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


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