Re: [PATCHSET v1 RFC liburing 0/6] Implement the kernel style return value

[Louvian Lyndal <louvianlyndal@gmail.com>]

On 9/29/21 5:16 PM, Ammar Faizi wrote:
> ### 3) How to deal syscalls
>
> We have 3 patches in this series to wrap the syscalls, they are:
>   - Add `liburing_mmap()` and `liburing_munmap()`
>   - Add `liburing_madvise()`
>   - Add `liburing_getrlimit()` and `liburing_setrlimit()`
>
> For `liburing_{munmap,madvise,getrlimit,setrlimit}`, they will return
> negative value of error code if error. They basically just return
> an int, so nothing to worry about.
>
> Special case is for pointer return value like `liburing_mmap()`. In
> this case we take the `include/linux/err.h` file from the Linux kernel
> source tree and use `IS_ERR()`, `PTR_ERR()`, `ERR_PTR()` to deal with
> it.
>
>
> ### 4) How can this help to support no libc environment?
>
> When this kernel style return value gets adapted on liburing, we will
> start working on raw syscall directly written in Assembly (arch
> dependent).
>
> Me (Ammar Faizi) will start kicking the tires from x86-64 arch.
> Hopefully we will get support for other architectures as well.
>
> The example of liburing syscall wrapper may look like this:
>
> ```c
> void *liburing_mmap(void *addr, size_t length, int prot, int flags,
>                     int fd, off_t offset)
> {      
> #ifdef LIBURING_NOLIBC
>         /*
>          * This is when we build without libc.
>          *
>          * Assume __raw_mmap is the syscall written in ASM.
>          *
>          * The return value is directly taken from the syscall
>          * return value.
>          */
>         return __raw_mmap(addr, length, prot, flags, fd, offset);
> #else
>         /*
>          * This is when libc exists.
>          */
>         void *ret;
>
>         ret = mmap(addr, length, prot, flags, fd, offset);
>         if (ret == MAP_FAILED)
>                 ret = ERR_PTR(-errno);
>
>         return ret;
> #endif
> }
> ```

This will add extra call just to wrap the libc. Consider to static
inline them?

For libc they just check the retval, if it's -1 then return -errno. If
they are inlined, they are ideally identical with the previous version.

Besides they are all internal functions. I don't see why should we
pollute global scope with extra wrappers.

Regards,

--
Louvian Lyndal