[PATCH v4] linux: Add openat2 (BZ 31664)

[Florian Weimer]

* Paul Eggert:

> On 2025-08-26 06:03, Aleksa Sarai wrote:
>> Is removing const something we can do safely? I was under the impression
>> removing const from a function prototype can break building working
>> programs? (Even if they wouldn't be using the feature.)
>
> How so? Can you give a sample call that would break?

Starting with GCC 14, the last two lines are reported as containing
errors:

void f(char *);
void fc(const char *);

void (*pf)(const char *) = f;
void (*pfc)(char *) = fc;

>> But there is precedent for the CHECK_FIELDS approach
>> -- sched_setattr(2) and perf_event_open(2) both write to the size field
>
> None of these are in glibc. And this is a confusing design. And there
> is no good performance reason for it. Syscalls should instead use the
> POSIX standard model of sigaction etc.

I don't think sigaction is a good example because it is not extensible.
POSIX examples for extensible interfaces are posix_spawn and
pthread_attr_t, where each attribute ends up with two functions.  That
approach isn't great, either.

> The glibc API can have two arguments even if the Linux syscall is
> poorly designed and has just one.

If we don't pass through the system call interface unchanged, past
experience tells us that this causes problems in the future.

Thanks,
Florian