On Wed, Sep 28, 2022 at 12:02 PM Pavel Begunkov <asml.silence@xxxxxxxxx> wrote:
>
> On 9/28/22 10:50, Ben Noordhuis wrote:
> > I'm trying to chain accept+read but it's not working.
> >
> > My code looks like this:
> >
> > *sqe1 = (struct io_uring_sqe){
> > .opcode = IORING_OP_ACCEPT,
> > .flags = IOSQE_IO_LINK,
> > .fd = listenfd,
> > .file_index = 42, // or 42+1
> > };
> > *sqe2 = (struct io_uring_sqe){
> > .opcode = IORING_OP_READ,
> > .flags = IOSQE_FIXED_FILE,
> > .addr = (u64) buf,
> > .len = len,
> > .fd = 42,
> > };
> > submit();
> >
> > Both ops fail immediately; accept with -ECANCELED, read with -EBADF,
> > presumably because fixed fd 42 doesn't exist at the time of submission.
> >
> > Would it be possible to support this pattern in io_uring or are there
> > reasons for why things are the way they are?
>
> It should already be supported. And errors look a bit odd, I'd rather
> expect -EBADF or some other for accept and -ECANCELED for the read.
> Do you have a test program / reporoducer? Hopefully in C.
Of course, please see below. Error handling elided for brevity. Hope
I'm not doing anything stupid.
For me it immediately prints this:
0 res=-125
1 res=-9
Some observations:
- it's not included in the test case but I can tell from the user_data
field the -EBADF is the read op
- replacing IORING_OP_READ with e.g. IORING_OP_NOP makes it work
(accepts a connection)
- once the fd has been installed, I can successfully chain
IOSQE_FIXED_FILE read&write ops
I'm primarily testing against a 5.15 kernel. Is this something that's
been fixed since? I went through the commit history but I didn't find
anything relevant.
---
#include <linux/io_uring.h>
#include <netinet/in.h>
#include <stdatomic.h>
#include <stdio.h>
#include <string.h>
#include <sys/mman.h>
#include <sys/syscall.h>
#include <unistd.h>
int main(void) {
struct sockaddr_in sin = {
.sin_family = AF_INET,
.sin_addr = (struct in_addr){ htonl(INADDR_ANY) },
.sin_port = htons(9000),
};
int listenfd = socket(AF_INET, SOCK_STREAM|SOCK_CLOEXEC, 0);
(void) bind(listenfd, (struct sockaddr *) &sin, sizeof(sin));
(void) listen(listenfd, 128);
struct io_uring_params p = {};
int ringfd = syscall(__NR_io_uring_setup, 32, &p);
int files[64]; memset(files, -1, sizeof(files));
syscall(__NR_io_uring_register,
ringfd, IORING_REGISTER_FILES, files, 64);
__u8 *sq = mmap(0, p.sq_off.array + p.sq_entries * sizeof(__u32),
PROT_READ|PROT_WRITE, MAP_SHARED|MAP_POPULATE,
ringfd, IORING_OFF_SQ_RING);
__u8 *cq = mmap(
0, p.cq_off.cqes + p.cq_entries * sizeof(struct io_uring_cqe),
PROT_READ|PROT_WRITE, MAP_SHARED|MAP_POPULATE,
ringfd, IORING_OFF_CQ_RING);
struct io_uring_sqe *sqe = mmap(0, p.sq_entries * sizeof(*sqe),
PROT_READ|PROT_WRITE, MAP_SHARED|MAP_POPULATE,
ringfd, IORING_OFF_SQES);
__u32 *sqtail = (__u32 *) (sq + p.sq_off.tail);
__u32 *sqarray = (__u32 *) (sq + p.sq_off.array);
struct io_uring_cqe *cqe =
(struct io_uring_cqe *) (cq + p.cq_off.cqes);
sqe[0] = (struct io_uring_sqe){
.opcode = IORING_OP_ACCEPT,
.flags = IOSQE_ASYNC|IOSQE_IO_LINK,
.fd = listenfd,
.file_index = 42,
};
char buf[256];
sqe[1] = (struct io_uring_sqe){
.opcode = IORING_OP_READ,
.flags = IOSQE_FIXED_FILE,
.fd = 42,
.len = sizeof(buf),
.addr = (__u64) buf,
};
sqarray[0] = 0; sqarray[1] = 1;
atomic_store((atomic_uint *) sqtail, 2);
int n = syscall(__NR_io_uring_enter,
ringfd, 2, 1, IORING_ENTER_GETEVENTS, 0, 0);
for (int i = 0; i < n; i++) printf("%d res=%d\n", i, cqe[i].res);
return 0;
}
