Weird readv behavior on 5.5 kernel

[twissel]

On 5.5 kernel in case if file size is less than iovec size, cqe.res will be equal to 0, On 5.4 kernel cqe.res will contain correct number of bytes read

Sample code:

#include <errno.h>
#include <stdio.h>
#include <unistd.h>
#include <stdlib.h>
#include <string.h>
#include <fcntl.h>
#include <sys/types.h>
#include <sys/poll.h>

#include "liburing.h"

#define BUF_SIZE 4096
#define FILE_SIZE 1024

static int create_file(const char *file)
{
	ssize_t ret;
	char *buf;
	int fd;

	buf = malloc(FILE_SIZE);
	memset(buf, 0xaa, FILE_SIZE);

	fd = open(file, O_WRONLY | O_CREAT, 0644);
	if (fd < 0) {
		perror("open file");
		return 1;
	}
	ret = write(fd, buf, FILE_SIZE);
	close(fd);
	return ret != FILE_SIZE;
}

int main(int argc, char* argv[]) {
    int ret, fd;
    struct io_uring ring;
    struct io_uring_sqe *sqe;
    struct io_uring_cqe *cqe;
    struct iovec vec;

    vec.iov_base = malloc(BUF_SIZE);
    vec.iov_len = BUF_SIZE;

    if (create_file(".basic-r")) {
		fprintf(stderr, "file creation failed\n");
		return 1;
	}

    fd = open(".basic-r", O_RDONLY);
	if (fd < 0) {
		perror("file open");
		return 1;
	}

    ret = io_uring_queue_init(32, &ring, 0);
	if (ret)
		return ret;


    sqe = io_uring_get_sqe(&ring);
    if (!sqe) {
			fprintf(stderr, "sqe get failed\n");
			return 1;
	}

    io_uring_prep_readv(sqe, fd, &vec, 1, 0);
    
    ret = io_uring_submit(&ring);
    if (ret != 1) {
        return 1;
    }

    ret = io_uring_wait_cqes(&ring, &cqe, 1, 0, 0);
    if (ret) {
        return 1;
    }

    fprintf(stderr, "cqe res %d", cqe->res);
    io_uring_cqe_seen(&ring, cqe);
    return 0;
}

[axboe]

Good catch, let me take a look at that.

[axboe]

https://git.kernel.dk/cgit/linux-block/commit/?h=io_uring-5.5&id=eacc6dfaea963ef61540abb31ad7829be5eff284

[axboe]

Adopted the test program as a real test case:

https://git.kernel.dk/cgit/liburing/commit/?id=46f92335793e70363c296b4c997da7a808959a49

[twissel]

Cool, thanks for the fix

[axboe]

No problem, thanks for the report!