[PATCHSET v4 RFC liburing 0/4] Implement the kernel style return value

* [PATCHSET v4 RFC liburing 0/4] Implement the kernel style return value
@ 2021-10-03 10:17 Ammar Faizi
  2021-10-03 10:17 ` [PATCH v4 RFC liburing 1/3] src/syscall: Wrap `errno` for `__sys_io_uring_{register,setup,enter{2,}}` Ammar Faizi
                   ` (3 more replies)
  0 siblings, 4 replies; 7+ messages in thread
From: Ammar Faizi @ 2021-10-03 10:17 UTC (permalink / raw)
  To: Jens Axboe, Pavel Begunkov
  Cc: io-uring Mailing List, Bedirhan KURT, Louvian Lyndal

This is the v4 of RFC to implement the kernel style return value for
liburing.

The main purpose of these changes is to make it possible to remove
the dependency of `errno` variable in the liburing C sources. If we
can land this on liburing, we will start working on adding support
build liburing without libc.

Currently, we expose these functions to userland:
  1) `__sys_io_uring_register`
  2) `__sys_io_uring_setup`
  3) `__sys_io_uring_enter2`
  4) `__sys_io_uring_enter`

The tests in `test/io_uring_{enter,register,setup}.c` are the examples
of it. Since the userland needs to check the `errno` value to use them
properly, this means those functions always depend on libc. So we
cannot change their behavior. Don't touch them all, this ensures the
changes only affect liburing internal and no visible functionality
changes for the users.

Then we introduce new functions with the same name (with extra
underscore as prefix, 4 underscores):
  1) `____sys_io_uring_register`
  2) `____sys_io_uring_setup`
  3) `____sys_io_uring_enter2`
  4) `____sys_io_uring_enter`

These functions do not use `errno` variable *on the caller*, they use
the kernel style return value (return a negative value of error code
when errors).

These functions are defined as `static inline` in `src/syscall.h`.
They are just a wrapper to make sure liburing internal sources do not
touch `errno` variable from C files directly. We need to make C files
not to touch the `errno` variable to support build without libc.

To completely remove the `errno` variable dependency from liburing C
files. We wrap all syscalls in a kernel style return value as well.

Currently we have 5 other syscalls in liburing. We wrapped all of
them as these 5 functions:
  1) `uring_mmap`
  2) `uring_munmap`
  3) `uring_madvise`
  4) `uring_getrlimit`
  5) `uring_setrlimit`

All of them are `static inline` and will return a negative value of
error code in case error happens.

Extra new helpers:
  1) `ERR_PTR()`
  2) `PTR_ERR()`
  3) `IS_ERR()`

These helpers are used to deal with syscalls that return a pointer.
Currently only `uring_mmap()` that depends on these.

If you want a git repository to test these patches, you can pull from:

    git://github.com/ammarfaizi2/liburing.git tags/kernel-style-retval-v4

Please review and comment.
----------------------------------------------------------------
Changes from v1 to v2:
- Make all wrapper functions be `static inline`, so they don't pollute
  the global scope.
- Reduce the number of patches. Now we only have 4 patches (it was 6).

Changes from v2 to v3:
- Remove duplicate Signed-off-by.

Changes from v3 to v4 (this thread):
- Fix unintentional logic change [1].
- Don't take the kernel header file to have extra helpers (ERR_PTR,
  PTR_ERR, IS_ERR). Write our own helpers with the same principles [2].
- Change the wrapper prefix, it was
  `liburing_{mmap,munmap,madvise,{get,set}rlimit}`, now we use
  `uring_{mmap,munmap,madvise,{get,set}rlimit}` to make it more
  compact and consistent (as we already have `uring_{{un,}likely})`.
- Reduce the number of patches. Now we only have 3 patches (it was 4).

Link: [GH Issue] https://github.com/axboe/liburing/issues/443
Link: [v1] https://lore.kernel.org/io-uring/20210929101606.62822-1-ammar.faizi@students.amikom.ac.id/T/
Link: [v2] https://lore.kernel.org/io-uring/20211002012817.107517-1-ammar.faizi@students.amikom.ac.id/T/
Link: [v3] https://lore.kernel.org/io-uring/20211002014829.109096-1-ammar.faizi@students.amikom.ac.id/T/

#Ref
Link: [1] https://lore.kernel.org/io-uring/d760c684-8175-6647-01c5-f0107b6685c6@gmail.com/
Link: [2] https://github.com/axboe/liburing/issues/446
----------------------------------------------------------------
Ammar Faizi (3):
      src/syscall: Wrap `errno` for `__sys_io_uring_{register,setup,enter{2,}}`
      src/{queue,register,setup}: Don't use `__sys_io_uring*`
      Wrap all syscalls in a kernel style return value

 src/queue.c    |  28 +++----
 src/register.c | 197 +++++++++++++++--------------------------------
 src/setup.c    |  57 +++++++-------
 src/syscall.c  |  43 +----------
 src/syscall.h  | 141 +++++++++++++++++++++++++++++++++
 5 files changed, 247 insertions(+), 219 deletions(-)

-- 
Ammar Faizi

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