linux-kernel - Re: [PATCH 2/4] selftest/vm: verify remap destination address in mremap

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Date:   Thu, 14 Apr 2022 15:47:46 -0600
From:   Shuah Khan <skhan@...uxfoundation.org>
To:     Sidhartha Kumar <sidhartha.kumar@...cle.com>, shuah@...nel.org,
        akpm@...ux-foundation.org
Cc:     linux-mm@...ck.org, linux-kselftest@...r.kernel.org,
        linux-kernel@...r.kernel.org,
        Shuah Khan <skhan@...uxfoundation.org>
Subject: Re: [PATCH 2/4] selftest/vm: verify remap destination address in
 mremap_test

On 4/14/22 11:15 AM, Sidhartha Kumar wrote:
> Because mremap does not have a NOREPLACE flag,
> it can destroy existing mappings. This can
> cause a segfault if regions such as text are
> destroyed.

Please explain the reason for segfault.

Add a blank line here. Makes it easier to read.

Verify the requested mremap destination
> address does not overlap any existing mappings
> by using mmap's FIXED_NOREPLACE flag and checking

Spell this out fully - MAP_FIXED_NOREPLACE
> for the EEXIST error code. Keep incrementing the
> destination address until a valid mapping is found
> or max address is reached.
> 

Essentially mremap() doesn't check for overlaps and removes
or overwrites existing mappings? The way you are fixing it
is by verifying by calling mremap() with MAP_FIXED_NOREPLACE
flag and check for EEXIST.

What happens when max address is reached?

Same comment on # of chars per line in commit log. Also

> Signed-off-by: Sidhartha Kumar <sidhartha.kumar@...cle.com>
> ---
>   tools/testing/selftests/vm/mremap_test.c | 36 ++++++++++++++++++++++++
>   1 file changed, 36 insertions(+)
> 
> diff --git a/tools/testing/selftests/vm/mremap_test.c b/tools/testing/selftests/vm/mremap_test.c
> index 58600fee4b81..98e9cff34aa7 100644
> --- a/tools/testing/selftests/vm/mremap_test.c
> +++ b/tools/testing/selftests/vm/mremap_test.c
> @@ -10,6 +10,7 @@
>   #include <string.h>
>   #include <sys/mman.h>
>   #include <time.h>
> +#include <limits.h>
>   
>   #include "../kselftest.h"
>   
> @@ -65,6 +66,34 @@ enum {
>   	.expect_failure = should_fail				\
>   }
>   
> +/*
> + * Returns 0 if the requested remap region overlaps with an
> + * existing mapping (e.g text, stack) else returns 1.
> + */
> +static int remap_region_valid(void *addr, unsigned long long size)

This returns bool 0 (false) 1 (true)

Please name the routine - is_remap_region_valid() and change it to
return bool.

> +{
> +	void *remap_addr = NULL;
> +	int ret = 1;
> +
> +	if ((unsigned long long) addr > ULLONG_MAX - size) {
> +		ksft_print_msg("Can't find a valid region to remap to\n");

Change it to "Couldn't" - also this message doesn't look right. We hav't
looked for valid region yet and it just exceeds the limits?


> +		exit(KSFT_SKIP);> +	}
> +
> +	/* Use MAP_FIXED_NOREPLACE flag to ensure region is not mapped */
> +	remap_addr = mmap(addr, size, PROT_READ | PROT_WRITE,
> +			MAP_FIXED_NOREPLACE | MAP_ANONYMOUS | MAP_SHARED,
> +			-1, 0);

Alignment should match open parenthesis here and in other places. Makes it
easier to read the code.

> +	if (remap_addr == MAP_FAILED) {
> +		if (errno == EEXIST)
> +			ret = 0;
> +	} else {
> +		munmap(remap_addr, size);
> +	}
> +
> +	return ret;
> +}
> +
>   /* Returns mmap_min_addr sysctl */
>   static unsigned long long get_mmap_min_addr(void)
>   {
> @@ -180,6 +209,13 @@ static long long remap_region(struct config c, unsigned int threshold_mb,
>   	if (!((unsigned long long) addr & c.dest_alignment))
>   		addr = (void *) ((unsigned long long) addr | c.dest_alignment);
>   
> +	/* Don't destroy existing mappings unless expected to overlap */
> +	while (!remap_region_valid(addr, c.region_size)) {
> +		if (c.overlapping)
> +			break;
> +		addr += c.src_alignment;
> +	}
> +
>   	clock_gettime(CLOCK_MONOTONIC, &t_start);
>   	dest_addr = mremap(src_addr, c.region_size, c.region_size,
>   			MREMAP_MAYMOVE|MREMAP_FIXED, (char *) addr);
> 

thanks,
-- Shuah