Q: arch/mips && __secure_computing(sd) with sd != NULL

From: Oleg Nesterov
Date: Sat Jan 18 2025 - 11:23:22 EST


>From include/linux/seccomp.h

#ifdef CONFIG_HAVE_ARCH_SECCOMP_FILTER
extern int __secure_computing(const struct seccomp_data *sd);

in this case sd can be NULL, and only arch/mips/kernel/ptrace.c uses sd != NULL

#else
static inline int __secure_computing(const struct seccomp_data *sd)
{
secure_computing_strict(sd->nr);
return 0;
}

in this case __secure_computing(NULL) will crash. This looks confusing.
And unnecessary.

Now lets look at arch/mips/kernel/ptrace.c:syscall_trace_enter()

#ifdef CONFIG_SECCOMP
if (unlikely(test_thread_flag(TIF_SECCOMP))) {
int ret, i;
struct seccomp_data sd;
unsigned long args[6];

sd.nr = current_thread_info()->syscall;
sd.arch = syscall_get_arch(current);
syscall_get_arguments(current, regs, args);
for (i = 0; i < 6; i++)
sd.args[i] = args[i];
sd.instruction_pointer = KSTK_EIP(current);

ret = __secure_computing(&sd);
if (ret == -1)
return ret;
}
#endif

Why? arch/mips/Kconfig selects HAVE_ARCH_SECCOMP_FILTER so it can just do

#ifdef CONFIG_SECCOMP
if (unlikely(test_thread_flag(TIF_SECCOMP))) {
int ret = __secure_computing(NULL);
if (ret == -1)
return ret;
}
#endif

and rely on populate_seccomp_data() and sd == NULL checks in __secure_computing()
path ?

And given that arch/mips doesn't define CONFIG_GENERIC_ENTRY it can even do

int ret = secure_computing();
if (ret == -1)
return ret;

Did I miss something?

Note that if we change arch/mips we can do more cleanups, in particular
__secure_computing(sd) no longer needs the argument.

Oleg.