Re: [PATCH] io_uring/register: guard compat syscall with CONFIG_COMPAT

[Jeff Moyer <jmoyer@redhat.com>]

Hi, Jens,

Jens Axboe <axboe@kernel.dk> writes:

> Add compat.h include to avoid a potential build issue:
>
> io_uring/register.c:281:6: error: call to undeclared function 'in_compat_syscall'; ISO C99 and later do not support implicit function declarations [-Werror,-Wimplicit-function-declaration]
>
> if (in_compat_syscall()) {
>     ^
> 1 warning generated.
> io_uring/register.c:282:9: error: call to undeclared function 'compat_get_bitmap'; ISO C99 and later do not support implicit function declarations [-Werror,-Wimplicit-function-declaration]
> ret = compat_get_bitmap(cpumask_bits(new_mask),
>       ^
>
> Fixes: c43203154d8a ("io_uring/register: move io_uring_register(2) related code to register.c")
> Reported-by: Manu Bretelle <chantra@meta.com>
> Signed-off-by: Jens Axboe <axboe@kernel.dk>
>
> ---
>
> diff --git a/io_uring/register.c b/io_uring/register.c
> index 708dd1d89add..5e62c1208996 100644
> --- a/io_uring/register.c
> +++ b/io_uring/register.c
> @@ -14,6 +14,7 @@
>  #include <linux/slab.h>
>  #include <linux/uaccess.h>
>  #include <linux/nospec.h>
> +#include <linux/compat.h>
>  #include <linux/io_uring.h>
>  #include <linux/io_uring_types.h>

This makes sense to me, but I wasn't able to reproduce that build error
after disabling CONFIG_COMPAT.

> @@ -278,13 +279,14 @@ static __cold int io_register_iowq_aff(struct io_ring_ctx *ctx,
>  	if (len > cpumask_size())
>  		len = cpumask_size();
>  
> -	if (in_compat_syscall()) {
> +#ifdef CONFIG_COMPAT
> +	if (in_compat_syscall())

I don't think this is needed.

linux/compat.h:
...
#else /* !CONFIG_COMPAT */

#define is_compat_task() (0)
/* Ensure no one redefines in_compat_syscall() under !CONFIG_COMPAT */
#define in_compat_syscall in_compat_syscall
static inline bool in_compat_syscall(void) { return false; }

Isn't the code fine as-is?

-Jeff

>  		ret = compat_get_bitmap(cpumask_bits(new_mask),
>  					(const compat_ulong_t __user *)arg,
>  					len * 8 /* CHAR_BIT */);
> -	} else {
> +	else
> +#endif
>  		ret = copy_from_user(new_mask, arg, len);
> -	}
>  
>  	if (ret) {
>  		free_cpumask_var(new_mask);