Memory does not get allocated with the MAP_ANONYMOUS and MAP_SHARED_VALIDATE flag in mmap()

Question

I am trying to allocate memory blocks with MAP_ANONYMOUS flag, but it is not creating any memory blocks along with MAP_SHARED_VALIDATE flag, but MAP_ANONYMOUS with MAP_PRIVATE or MAP_SHARED flags creates the blocks of memory. Could someone explain why this happens.

#include <stdio.h>
#include <string.h>
#include <errno.h>
#include <sys/mman.h>
#include <unistd.h>

int main(void)
{
  size_t size = getpagesize();

  errno = 0;
  void *first = mmap(0, size, PROT_READ|PROT_WRITE,
                     MAP_ANONYMOUS|MAP_SHARED_VALIDATE, -1, 0);
  printf("first: %p %s\n", first, strerror(errno));

  errno = 0;
  void *second = mmap(0, size, PROT_READ|PROT_WRITE,
                      MAP_ANONYMOUS|MAP_SHARED, -1, 0);
  printf("second: %p %s\n", second, strerror(errno));

  return 0;
}

has been observed to print, on both Linux 4.19 and Linux 5.8,

first: 0xffffffffffffffff Invalid argument
second: 0x7f56b274d000 Success

The flags seem to be getting passed down to the kernel accurately...

$ strace -e trace=mmap ./a.out 2>&1 | tail -n5
mmap(NULL, 4096, PROT_READ|PROT_WRITE, MAP_SHARED_VALIDATE|MAP_ANONYMOUS, -1, 0) = -1 EINVAL (Invalid argument)
mmap(NULL, 4096, PROT_READ|PROT_WRITE, MAP_SHARED|MAP_ANONYMOUS, -1, 0) = 0x7fd3145bb000
first: 0xffffffffffffffff Invalid argument
second: 0x7fd3145bb000 Success
+++ exited with 0 +++

In your code sample, there is no difference between mmap calls (but then I'm not sure what you wanted to showcase with 2 calls). Also, your printf calls access out of bounds memory, as you haven't provided an argument for %p format. If mmap returns -1, you should look at errno and see what the error is first. — Pawel Veselov
Note that the second call to mmap may change the value of errno, so you need to look at it immediately after each mmap call, something like: uint8_t *first = mmap(...); printf("first: %p %s\n", (void *)first, strerror(errno)); (repeat for second). — zwol
I can reproduce the behavior reported (mmap failing, not the segfault) on Linux 5.8 and I have absolutely no idea why. errno is EINVAL. As best I can tell from the manpage, MAP_SHARED_VALIDATE | MAP_ANONYMOUS should be a valid combination of flags. — zwol
Does your kernel support MAP_SHARED_VALIDATE? The man page says "since Linux 4.15". — Ian Abbott

Ian Abbott Ian Abbott · Accepted Answer · 2020-10-14T14:01:48

Looking at do_mmap in linux/mm/mmap.c (kernel version 5.9), MAP_SHARED_VALIDATE only seems to be supported for file-backed mappings (see the if (file) and else sections). I do not know if that is a bug or if it is intentional.

EDIT: I have submitted a bug report.

Memory does not get allocated with the MAP_ANONYMOUS and MAP_SHARED_VALIDATE flag in mmap()

1 Answers