Hi, Willy
On Sat, Aug 12, 2023 at 05:51:53AM +0800, Zhangjin Wu wrote:
As reported and suggested by Willy, the inline __sysret() helper introduces three types of conversions and increases the size:
(1) the "unsigned long" argument to __sysret() forces a sign extension from all sys_* functions that used to return 'int'
(2) the comparison with the error range now has to be performed on a 'unsigned long' instead of an 'int'
(3) the return value from __sysret() is a 'long' (note, a signed long) which then has to be turned back to an 'int' before being returned by the caller to satisfy the caller's prototype.
To fix up this, firstly, let's use macro instead of inline function to preserves the input type and avoids these useless conversions (1), (3).
Secondly, since all of the sys_* functions have been converted to return integer, now, it is able to remove comparison to a 'unsigned long' -MAX_ERRNO (2) and restore the simple sign comparison as before.
(...)
+/* Syscall return helper, set errno as -ret when ret < 0 */ +#define __sysret(arg) \ +({ \
- __typeof__(arg) __ret = (arg); \
- if (__ret < 0) { \
SET_ERRNO(-__ret); \__ret = -1L; \- } \
- __ret; \
+})
Except that this now breaks brk(), mmap() and sbrk() by taking any value with MSB set as an error. Also you've re-introduced the problem you've faced with const. See my simplification in the other thread by using "?:" which does avoids any assignment.
Yeah, thanks for your explanation in this reply [1], the 'const' flag only triggers build error on the second 'assign' (__ret == -1L), the first 'assign' is a definition, it is not problematic. so, your "?:" method is a great idea to simply return without the second 'assign'.
Let's just roll brk(), mmap() and sbrk() to their original, working, definition:
static __attribute__((unused)) void *mmap(void *addr, size_t length, int prot, int flags, int fd, off_t offset) { void *ret = sys_mmap(addr, length, prot, flags, fd, offset); if ((unsigned long)ret >= -MAX_ERRNO) { SET_ERRNO(-(long)ret); ret = MAP_FAILED; } return ret; }
Agree, only left a suggestion here [2] about whether we can apply the 2nd patch instead of rolling them back, let's discuss it in [2] thread.
And we're done, you can then keep the simplified __sysret() macro for all other call places.
Now, this issue is near to the end ;-)
Thanks! Zhangjin ---
[1]: https://lore.kernel.org/lkml/20230813085140.GD8237@1wt.eu/#R [2]: https://lore.kernel.org/lkml/20230813132620.19411-1-falcon@tinylab.org/
Willy