[PATCH v2] riscv: entry: set a0 prior to syscall_enter_from_user_mode

[Celeste Liu]

On 2023/7/19 03:35, Björn Töpel wrote:
> Celeste Liu <coelacanthushex at gmail.com> writes:
> 
>> 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 9c2598d43510 ("riscv: entry: Save a0
>> prior syscall_enter_from_user_mode()").
>>
>> After analysing code, we think that regs->a0 = -ENOSYS should be advanced before
>> syscall_enter_from_user_mode to fix this problem. 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 executable. It covered a0 set by seccomp, so we always get
>> ENOSYS when match seccomp RET_ERRNO rule.
> 
> Isn't something like...
> 
> diff --git a/arch/riscv/kernel/traps.c b/arch/riscv/kernel/traps.c
> index f910dfccbf5d..15a8b4898a6c 100644
> --- a/arch/riscv/kernel/traps.c
> +++ b/arch/riscv/kernel/traps.c
> @@ -297,7 +297,7 @@ 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)) {
> -		ulong syscall = regs->a7;
> +		long syscall = regs->a7;
>  
>  		regs->epc += 4;
>  		regs->orig_a0 = regs->a0;
> @@ -306,7 +306,7 @@ asmlinkage __visible __trap_section void do_trap_ecall_u(struct pt_regs *regs)
>  
>  		syscall = syscall_enter_from_user_mode(regs, syscall);
>  
> -		if (syscall < NR_syscalls)
> +		if (syscall > -1 && syscall < NR_syscalls)
>  			syscall_handler(regs, syscall);
>  		else
>  			regs->a0 = -ENOSYS;
> 
> 
> ...easier to read?
> 
> 
> Björn

It seems that your change turn it back to the beginning. If syscall == -1,
it is supposed to go neither first nor else branch. It should do NOTHING.
However it was still a great idea. It may be better to use a set of if-statement
to clarify the logic.

diff --git a/arch/riscv/kernel/traps.c b/arch/riscv/kernel/traps.c
index f910dfccbf5d2..d0bd725244594 100644
--- a/arch/riscv/kernel/traps.c
+++ b/arch/riscv/kernel/traps.c
@@ -306,7 +306,9 @@ asmlinkage __visible __trap_section void do_trap_ecall_u(struct pt_regs *regs)
 
 		syscall = syscall_enter_from_user_mode(regs, syscall);
 
-		if (syscall < NR_syscalls)
+		if (syscall == -1)
+			// Do nothing
+		else if (syscall < NR_syscalls)
 			syscall_handler(regs, syscall);
 		else
 			regs->a0 = -ENOSYS;