[PATCH v3] riscv: entry: set a0 = -ENOSYS only when syscall != -1

[Celeste Liu <coelacanthushex@gmail.com>]

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 instead. We got same result with commit 9c2598d43510 ("riscv: entry:
Save a0 prior syscall_enter_from_user_mode()").

After analysing code, we think that regs->a0 = -ENOSYS should only be executed
when syscall != -1 In __seccomp_filter, when seccomp rejected this syscall with
specified errno, they will set a0 to return number as syscall ABI, and then
return -1. This return number is finally pass as return number of
syscall_enter_from_user_mode, and then is compared with NR_syscalls after
converted to ulong (so it will be ULONG_MAX). The condition
syscall < NR_syscalls will always be false, so regs->a0 = -ENOSYS is always
executed. It covered a0 set by seccomp, so we always get ENOSYS when match
seccomp RET_ERRNO rule.

Fixes: f0bddf50586d ("riscv: entry: Convert to generic entry")
Reported-by: Felix Yan <felixonmars@archlinux.org>
Co-developed-by: Ruizhe Pan <c141028@gmail.com>
Signed-off-by: Ruizhe Pan <c141028@gmail.com>
Co-developed-by: Shiqi Zhang <shiqi@isrc.iscas.ac.cn>
Signed-off-by: Shiqi Zhang <shiqi@isrc.iscas.ac.cn>
Signed-off-by: Celeste Liu <CoelacanthusHex@gmail.com>
Tested-by: Felix Yan <felixonmars@archlinux.org>
---

v2 -> v3: use if-statement instead of set default value,
          clarify the type of syscall
v1 -> v2: added explanation on why always got ENOSYS

 arch/riscv/kernel/traps.c | 6 +++++-
 1 file changed, 5 insertions(+), 1 deletion(-)

diff --git a/arch/riscv/kernel/traps.c b/arch/riscv/kernel/traps.c
index f910dfccbf5d2..5cef728745420 100644
--- a/arch/riscv/kernel/traps.c
+++ b/arch/riscv/kernel/traps.c
@@ -297,6 +297,10 @@ asmlinkage __visible __trap_section void do_trap_break(struct pt_regs *regs)
 asmlinkage __visible __trap_section void do_trap_ecall_u(struct pt_regs *regs)
 {
 	if (user_mode(regs)) {
+		/*
+		 * Convert negative numbers to very high and thus out of range
+		 * numbers for comparisons.
+		 */
 		ulong syscall = regs->a7;
 
 		regs->epc += 4;
@@ -308,7 +312,7 @@ asmlinkage __visible __trap_section void do_trap_ecall_u(struct pt_regs *regs)
 
 		if (syscall < NR_syscalls)
 			syscall_handler(regs, syscall);
-		else
+		else if ((long)syscall != -1L)
 			regs->a0 = -ENOSYS;
 
 		syscall_exit_to_user_mode(regs);
-- 
2.41.0