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SubjectRe: [RFC PATCH] Take mmap_min_addr into account while choosing unmapped address for x86-64.
From
Date
> 
> mmap_min_addr handling is a bit mess... As you say, we would return
> EPERM rather than ENOMEM which can be confusing but depleting the
> address space like that is quite unlikely on 64b unless I am missing.
> It is good to be in sync here with the generic implementation though,
> IMO.
>

If we take a look on mm/mmap.c:
#ifndef HAVE_ARCH_UNMAPPED_AREA_TOPDOWN
unsigned long
arch_get_unmapped_area_topdown(

if (len > TASK_SIZE - mmap_min_addr)
return -ENOMEM;

info.low_limit = max(PAGE_SIZE, mmap_min_addr);

And this one looks like a generic implementation.
But for many other architectures like arch/parisc/kernel/sys_parisc.c
or arch/x86/kernel/sys_x86_64.c

info.low_limit = PAGE_SIZE;

What is looks like an issue for me.

Here is C code could be used as test-case:

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

int main() {
char buffer[1024];

unsigned long len = 1ULL << 46;
while(len) {
void *ptr = mmap(4096, len, 0, MAP_PRIVATE | MAP_ANONYMOUS , -1, 0);
if (ptr == MAP_FAILED) {
if (errno == EPERM)
break;
if (errno == ENOMEM) {
len >>= 1;
continue;
}
return -1;
}
}
if (errno == EPERM) {
printf("Test failed, you have wrong ret code EPERM\n");
sprintf(buffer, "cat /proc/%d/maps", getpid());
system(buffer);
return -1;
}
return 0;
}


>>
>> + if (addr < mmap_min_addr)
>> + return false;
>> +
>> return (addr > DEFAULT_MAP_WINDOW) == (addr + len > DEFAULT_MAP_WINDOW);
>
> But is this one necessary? We do sanitze hint address before going to
> get_unmapped_address AFAIR.
>
I’m agree, looks like I was trying to fix something that already fine.

Thanks,
Ilya

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