install_special_mapping && vm_pgoff (Was: vvar, gup && coredump)

Oleg Nesterov
2015-03-16T19:03:53Z

On 03/12, Oleg Nesterov wrote:
>
> OTOH. We can probably add ->access() into special_mapping_vmops, this
> way __access_remote_vm() could work even if gup() fails ?

So I tried to think how special_mapping_vmops->access() can work, it
needs to rely on ->vm_pgoff.

But afaics this logic is just broken. Lets even forget about vvar vma
which uses remap_file_pages(). Lets look at "[vdso]" which uses the
"normal" pages.

The comment in special_mapping_fault() says

* special mappings have no vm_file, and in that case, the mm
* uses vm_pgoff internally.

Yes. But afaics mm/ doesn't do this correctly. So

* do not copy this code into drivers!

looks like a good recommendation ;)

I think that this logic is wrong even if ARRAY_SIZE(pages) == 1, but I am
not sure. But since vdso use 2 pages, it is trivial to show that this logic
is wrong. To verify, I changed show_map_vma() to expose pgoff even if !file,
but this test-case can show the problem too:

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

void *find_vdso_vaddr(void)
{
FILE *perl;
char buf[32] = {};

perl = popen("perl -e 'open STDIN,qq|/proc/@{[getppid]}/maps|;"
"/^(.*?)-.*vdso/ && print hex $1 while <>'", "r");
fread(buf, sizeof(buf), 1, perl);
fclose(perl);

return (void *)atol(buf);
}

#define PAGE_SIZE 4096

int main(void)
{
void *vdso = find_vdso_vaddr();
assert(vdso);

// of course they should differ, and they do so far
printf("vdso pages differ: %d\n",
!!memcmp(vdso, vdso + PAGE_SIZE, PAGE_SIZE));

// split into 2 vma's
assert(mprotect(vdso, PAGE_SIZE, PROT_READ) == 0);

// force another fault on the next check
assert(madvise(vdso, 2 * PAGE_SIZE, MADV_DONTNEED) == 0);

// now they no longer differ, the 2nd vm_pgoff is wrong
printf("vdso pages differ: %d\n",
!!memcmp(vdso, vdso + PAGE_SIZE, PAGE_SIZE));

return 0;
}

output:

vdso pages differ: 1
vdso pages differ: 0

And not only "split_vma" is wrong, I think that "move_vma" is not right too.
Note this check in copy_vma(),

/*
* If anonymous vma has not yet been faulted, update new pgoff
* to match new location, to increase its chance of merging.
*/
if (unlikely(!vma->vm_file && !vma->anon_vma)) {
pgoff = addr >> PAGE_SHIFT;
faulted_in_anon_vma = false;
}

I can easily misread this code. But it doesn't look right too. If vdso was cow'ed
(breakpoint installed by gdb) and sys_nremap()'ed, then the new pgoff will be wrong
too after, say, MADV_DONTNEED.

Or I am totally confused?

Oleg.