[PATCH] riscv: entry: set a0 prior to syscall_handler

[Celeste Liu]

When we test seccomp with 6.4 kernel, we found errno has wrong value.
If we deny NETLINK_AUDIT with EAFNOSUPPORT, after f0bddf50586d, we will
get ENOSYS. We got same result with 9c2598d43510 ("riscv: entry: Save a0
prior syscall_enter_from_user_mode()").

Compared with x86 and loongarch's implementation of this part of the
function, we think that regs->a0 = -ENOSYS should be advanced before
syscall_handler to fix this problem. We have written the following patch,
which can fix this problem after testing. But we don't know enough about
this part of the code to explain the root cause. Hope someone can find
a reasonable explanation. And we'd like to reword this commit message
according to the explanation in v2

Fixes: f0bddf50586d ("riscv: entry: Convert to generic entry")
Reported-by: Felix Yan <felixonmars at archlinux.org>
Co-developed-by: Ruizhe Pan <c141028 at gmail.com>
Signed-off-by: Ruizhe Pan <c141028 at gmail.com>
Co-developed-by: Shiqi Zhang <shiqi at isrc.iscas.ac.cn>
Signed-off-by: Shiqi Zhang <shiqi at isrc.iscas.ac.cn>
Signed-off-by: Celeste Liu <CoelacanthusHex at gmail.com>
Tested-by: Felix Yan <felixonmars at archlinux.org>
---
 arch/riscv/kernel/traps.c | 3 +--
 1 file changed, 1 insertion(+), 2 deletions(-)

diff --git a/arch/riscv/kernel/traps.c b/arch/riscv/kernel/traps.c
index f910dfccbf5d2..ccadb5ffd063c 100644
--- a/arch/riscv/kernel/traps.c
+++ b/arch/riscv/kernel/traps.c
@@ -301,6 +301,7 @@ asmlinkage __visible __trap_section 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);
 
@@ -308,8 +309,6 @@ asmlinkage __visible __trap_section void do_trap_ecall_u(struct pt_regs *regs)
 
 		if (syscall < NR_syscalls)
 			syscall_handler(regs, syscall);
-		else
-			regs->a0 = -ENOSYS;
 
 		syscall_exit_to_user_mode(regs);
 	} else {
-- 
2.41.0