Re: io_uring: memory accounting quirk with IORING_REGISTER_CLONE_BUFFERS

[Jens Axboe <axboe@kernel.dk>]

On 1/14/25 10:16 AM, Jann Horn wrote:
> Hi,
> 
> I noticed that io_uring's memory accounting behaves weirdly when
> IORING_REGISTER_CLONE_BUFFERS is used to clone buffers from uring
> instance A to uring instance B, where A and B use different MMs for
> accounting. If I first close uring instance A and then uring instance
> B, the pinned memory counters for uring instance B will be subtracted,
> even though the pinned memory was originally accounted through uring
> instance A; so the MM of uring instance B can end up with negative
> locked memory.
> 
> Here is a testcase:
> ```
> #define _GNU_SOURCE
> #include <err.h>
> #include <unistd.h>
> #include <stdio.h>
> #include <stdlib.h>
> #include <fcntl.h>
> #include <sys/syscall.h>
> #include <sys/mman.h>
> #include <sys/uio.h>
> #include <linux/io_uring.h>
> 
> /* for building with outdated kernel headers */
> #if 1
> enum {
>         IORING_REGISTER_SRC_REGISTERED  = (1U << 0),
>         IORING_REGISTER_DST_REPLACE     = (1U << 1),
> };
> struct io_uring_clone_buffers {
>         __u32   src_fd;
>         __u32   flags;
>         __u32   src_off;
>         __u32   dst_off;
>         __u32   nr;
>         __u32   pad[3];
> };
> #define IORING_REGISTER_CLONE_BUFFERS 30
> #endif
> 
> #define SYSCHK(x) ({          \
>   typeof(x) __res = (x);      \
>   if (__res == (typeof(x))-1) \
>     err(1, "SYSCHK(" #x ")"); \
>   __res;                      \
> })
> 
> #define NUM_SQ_PAGES 4
> static int uring_init(struct io_uring_sqe **sqesp, void **cqesp) {
>   struct io_uring_sqe *sqes = SYSCHK(mmap(NULL, NUM_SQ_PAGES*0x1000,
> PROT_READ|PROT_WRITE, MAP_SHARED|MAP_ANONYMOUS, -1, 0));
>   void *cqes = SYSCHK(mmap(NULL, NUM_SQ_PAGES*0x1000,
> PROT_READ|PROT_WRITE, MAP_SHARED|MAP_ANONYMOUS, -1, 0));
>   *(volatile unsigned int *)(cqes+4) = 64 * NUM_SQ_PAGES;
>   struct io_uring_params params = {
>     .flags = IORING_SETUP_NO_MMAP|IORING_SETUP_NO_SQARRAY,
>     .sq_off = { .user_addr = (unsigned long)sqes },
>     .cq_off = { .user_addr = (unsigned long)cqes }
>   };
>   int uring_fd = SYSCHK(syscall(__NR_io_uring_setup, /*entries=*/10, &params));
>   if (sqesp)
>     *sqesp = sqes;
>   if (cqesp)
>     *cqesp = cqes;
>   return uring_fd;
> }
> 
> int main(int argc, char **argv) {
>   if (argc == 1) {
>     int ring1 = uring_init(NULL, NULL);
>     SYSCHK(fcntl(ring1, F_SETFD, 0)); /* clear O_CLOEXEC */
>     char *bufmem = SYSCHK(mmap(NULL, 0x1000, PROT_READ|PROT_WRITE,
> MAP_PRIVATE|MAP_ANONYMOUS, -1, 0));
>     struct iovec reg_iov = { .iov_base = bufmem, .iov_len = 0x1000 };
>     SYSCHK(syscall(__NR_io_uring_register, ring1,
> IORING_REGISTER_BUFFERS, &reg_iov, 1));
>     char fd_str[100];
>     sprintf(fd_str, "%d", ring1);
>     execlp(argv[0], argv[0], fd_str, NULL);
>     err(1, "reexec");
>   } else if (argc == 2) {
>     int ring1 = atoi(argv[1]);
>     int ring2 = uring_init(NULL, NULL);
>     struct io_uring_clone_buffers arg = {
>       .src_fd = ring1,
>       .flags = 0,
>       .src_off = 0,
>       .dst_off = 0,
>       .nr = 1
>     };
>     SYSCHK(syscall(__NR_io_uring_register, ring2,
> IORING_REGISTER_CLONE_BUFFERS, &arg, 1));
>     close(ring1);
>     close(ring2);
>     system("cat /proc/$PPID/status");
>     return 0;
>   } else {
>     errx(1, "please run without any arguments");
>   }
> }
> ```
> 
> Result:
> ```
> $ gcc -o uring-buf-deaccount uring-buf-deaccount.c
> $ ./uring-buf-deaccount
> Name:   uring-buf-deacc
> Umask:  0002
> State:  S (sleeping)
> Tgid:   1162
> Ngid:   0
> Pid:    1162
> PPid:   968
> TracerPid:      0
> Uid:    1000    1000    1000    1000
> Gid:    1000    1000    1000    1000
> FDSize: 256
> Groups: 1000
> NStgid: 1162
> NSpid:  1162
> NSpgid: 1162
> NSsid:  968
> Kthread:        0
> VmPeak:     2540 kB
> VmSize:     2456 kB
> VmLck:         0 kB
> VmPin:  18446744073709551612 kB
> VmHWM:      1264 kB
> VmRSS:      1264 kB
> RssAnon:               0 kB
> RssFile:            1264 kB
> RssShmem:              0 kB
> [...]
> ```
> 
> Note the "VmPin:  18446744073709551612 kB", that's 0xfffffffffffffffc or -4.
> 
> This doesn't lead to anything particularly bad; it just means the
> memory usage accounting is off.
> 
> Commit 7cc2a6eadcd7 ("io_uring: add IORING_REGISTER_COPY_BUFFERS
> method") says that the intended usecase for
> IORING_REGISTER_CLONE_BUFFERS are thread pools; maybe a reasonable fix
> would be to just refuse IORING_REGISTER_CLONE_BUFFERS between rings
> with different accounting contexts (meaning different ->user or
> ->mm_account)? If that restriction seems acceptable, I'd write a patch
> for that.

I think adding that restriction would be fine. The intended use case is
sharing buffers anyway, which means they would need to be in the same
address space to begin with.

-- 
Jens Axboe