From: Willy Tarreau
Sent: 14 August 2023 13:10
Hi David,
On Mon, Aug 14, 2023 at 11:15:51AM +0000, David Laight wrote:
From: Zhangjin Wu
Sent: 14 August 2023 11:42
...
[...]
Sure it's not pretty, and I'd rather just go back to SET_ERRNO() to be honest, because we're there just because of the temptation to remove lines that were not causing any difficulties :-/
I think we can do something in-between and deal only with signed returns, and explicitly place the test for MAX_ERRNO on the two unsigned ones (brk and mmap). It should look approximately like this:
#define __sysret(arg) \ ({ \ __typeof__(arg) __sysret_arg = (arg); \ (__sysret_arg < 0) ? ({ /* error ? */ \ SET_ERRNO(-__sysret_arg); /* yes: errno != -ret */ \ ((__typeof__(arg)) -1); /* return -1 */ \
I'm pretty sure you don't need the explicit cast. (It would be needed for a pointer type.) Can you use __arg < ? SET_ERRNO(-__arg), -1 : __arg
Thinking, maybe it should be:
#define __sysret(syscall_fn_args) ({ __typeof__(syscall_fn_args) __rval = syscall_fn_args; __rval >= 0 ? __rval : SET_ERRNO(-__rval), -1; })
Yeah almost, since arg is necessarily signed in this version, it's just that I manually edited the previous macro in the mail and limited the amount of changes to what was necessary. It's just that SET_ERRNO only is an instruction, not an expression:
#define SET_ERRNO(v) do { errno = (v); } while (0)
Thus the return value doesn't even pass through it. That's why it was so much simpler before. The rationale behind this was to bring the ability to completely drop errno for programs where you didn't care about it. It's particularly interesting when you don't need any other data either as the program gets strunk from a complete section.
Actually something like:
#define SET_ERRNO(v) (errno = -(long)(v), __typeof__(v)-1)
seems to work and allows the errno assignment be removed. Also works for pointer types (after a different compare).
A quick check with godbolt doesn't show any sign extensions happening.
David
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