[PATCH] selftests: secretmem: Floor the memory size to the multiple of page_size

[PATCH] selftests: secretmem: Floor the memory size to the multiple of page_size

[Muhammad Usama Anjum]

The "locked-in-memory size" limit per process can be non-multiple of
page_size. The mmap() fails if we try to allocate locked-in-memory
with same size as the allowed limit if it isn't multiple of the
page_size because mmap() rounds off the memory size to be allocated
to next multiple of page_size.

Fix this by flooring the length to be allocated with mmap() to the
previous multiple of the page_size.

Fixes: 76fe17ef588a ("secretmem: test: add basic selftest for memfd_secret(2)")
Reported-by: "kernelci.org bot" <bot@kernelci.org>
Signed-off-by: Muhammad Usama Anjum <usama.anjum@collabora.com>
---
 tools/testing/selftests/mm/memfd_secret.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/tools/testing/selftests/mm/memfd_secret.c b/tools/testing/selftests/mm/memfd_secret.c
index 957b9e18c729..9b298f6a04b3 100644
--- a/tools/testing/selftests/mm/memfd_secret.c
+++ b/tools/testing/selftests/mm/memfd_secret.c
@@ -62,6 +62,9 @@ static void test_mlock_limit(int fd)
 	char *mem;
 
 	len = mlock_limit_cur;
+	if (len % page_size != 0)
+		len = (len/page_size) * page_size;
+
 	mem = mmap(NULL, len, prot, mode, fd, 0);
 	if (mem == MAP_FAILED) {
 		fail("unable to mmap secret memory\n");
-- 
2.42.0

Re: [PATCH] selftests: secretmem: Floor the memory size to the multiple of page_size

[Andrew Morton]

On Thu, 14 Dec 2023 15:19:30 +0500 Muhammad Usama Anjum <usama.anjum@collabora.com> wrote:

> The "locked-in-memory size" limit per process can be non-multiple of
> page_size. The mmap() fails if we try to allocate locked-in-memory
> with same size as the allowed limit if it isn't multiple of the
> page_size because mmap() rounds off the memory size to be allocated
> to next multiple of page_size.
> 
> Fix this by flooring the length to be allocated with mmap() to the
> previous multiple of the page_size.

I'd like to understand how this was noticed, what the ongoing effect
might be, etc.  To help decide which kernel version(s) need the patch.

> Fixes: 76fe17ef588a ("secretmem: test: add basic selftest for memfd_secret(2)")
> Reported-by: "kernelci.org bot" <bot@kernelci.org>

Which is one of the reasons we're now placing a Closes: tag after a
Reported-by:.

Re: [PATCH] selftests: secretmem: Floor the memory size to the multiple of page_size

[Muhammad Usama Anjum]

Hi Andrew,

On 12/15/23 12:40 AM, Andrew Morton wrote:
> On Thu, 14 Dec 2023 15:19:30 +0500 Muhammad Usama Anjum <usama.anjum@collabora.com> wrote:
> 
>> The "locked-in-memory size" limit per process can be non-multiple of
>> page_size. The mmap() fails if we try to allocate locked-in-memory
>> with same size as the allowed limit if it isn't multiple of the
>> page_size because mmap() rounds off the memory size to be allocated
>> to next multiple of page_size.
>>
>> Fix this by flooring the length to be allocated with mmap() to the
>> previous multiple of the page_size.
> 
> I'd like to understand how this was noticed, what the ongoing effect
> might be, etc.  To help decide which kernel version(s) need the patch.
This was getting triggered on KernelCI regularly because of different
ulimit settings which wasn't multiple of the page_size. Find logs here:
https://linux.kernelci.org/test/plan/id/657654bd8e81e654fae13532/ The bug
in was present from the time test was first added.
> 
>> Fixes: 76fe17ef588a ("secretmem: test: add basic selftest for memfd_secret(2)")
>> Reported-by: "kernelci.org bot" <bot@kernelci.org>
> 
> Which is one of the reasons we're now placing a Closes: tag after a
> Reported-by:.
I was looking for email report from KernelCI. But I didn't find it. Not
sure if we can do something like following:
Closes: https://linux.kernelci.org/test/plan/id/657654bd8e81e654fae13532/

> 
> 

-- 
BR,
Muhammad Usama Anjum

Re: [PATCH] selftests: secretmem: Floor the memory size to the multiple of page_size

[Mike Rapoport]

On Thu, Dec 14, 2023 at 03:19:30PM +0500, Muhammad Usama Anjum wrote:
> The "locked-in-memory size" limit per process can be non-multiple of
> page_size. The mmap() fails if we try to allocate locked-in-memory
> with same size as the allowed limit if it isn't multiple of the
> page_size because mmap() rounds off the memory size to be allocated
> to next multiple of page_size.
> 
> Fix this by flooring the length to be allocated with mmap() to the
> previous multiple of the page_size.
> 
> Fixes: 76fe17ef588a ("secretmem: test: add basic selftest for memfd_secret(2)")
> Reported-by: "kernelci.org bot" <bot@kernelci.org>
> Signed-off-by: Muhammad Usama Anjum <usama.anjum@collabora.com>
> ---
>  tools/testing/selftests/mm/memfd_secret.c | 3 +++
>  1 file changed, 3 insertions(+)
> 
> diff --git a/tools/testing/selftests/mm/memfd_secret.c b/tools/testing/selftests/mm/memfd_secret.c
> index 957b9e18c729..9b298f6a04b3 100644
> --- a/tools/testing/selftests/mm/memfd_secret.c
> +++ b/tools/testing/selftests/mm/memfd_secret.c
> @@ -62,6 +62,9 @@ static void test_mlock_limit(int fd)
>  	char *mem;
>  
>  	len = mlock_limit_cur;
> +	if (len % page_size != 0)
> +		len = (len/page_size) * page_size;
> +

With mlock limit smaller than a page we get zero length here and mmap will
fail with -EINVAL because of it.
In this case I think we can just skip the first mmap and only check that
mmaping more than mlock limit fails.

>  	mem = mmap(NULL, len, prot, mode, fd, 0);
>  	if (mem == MAP_FAILED) {
>  		fail("unable to mmap secret memory\n");
> -- 
> 2.42.0
> 

-- 
Sincerely yours,
Mike.

Re: [PATCH] selftests: secretmem: Floor the memory size to the multiple of page_size

[Muhammad Usama Anjum]

On 12/15/23 3:59 PM, Mike Rapoport wrote:
> On Thu, Dec 14, 2023 at 03:19:30PM +0500, Muhammad Usama Anjum wrote:
>> The "locked-in-memory size" limit per process can be non-multiple of
>> page_size. The mmap() fails if we try to allocate locked-in-memory
>> with same size as the allowed limit if it isn't multiple of the
>> page_size because mmap() rounds off the memory size to be allocated
>> to next multiple of page_size.
>>
>> Fix this by flooring the length to be allocated with mmap() to the
>> previous multiple of the page_size.
>>
>> Fixes: 76fe17ef588a ("secretmem: test: add basic selftest for memfd_secret(2)")
>> Reported-by: "kernelci.org bot" <bot@kernelci.org>
>> Signed-off-by: Muhammad Usama Anjum <usama.anjum@collabora.com>
>> ---
>>  tools/testing/selftests/mm/memfd_secret.c | 3 +++
>>  1 file changed, 3 insertions(+)
>>
>> diff --git a/tools/testing/selftests/mm/memfd_secret.c b/tools/testing/selftests/mm/memfd_secret.c
>> index 957b9e18c729..9b298f6a04b3 100644
>> --- a/tools/testing/selftests/mm/memfd_secret.c
>> +++ b/tools/testing/selftests/mm/memfd_secret.c
>> @@ -62,6 +62,9 @@ static void test_mlock_limit(int fd)
>>  	char *mem;
>>  
>>  	len = mlock_limit_cur;
>> +	if (len % page_size != 0)
>> +		len = (len/page_size) * page_size;
>> +
> 
> With mlock limit smaller than a page we get zero length here and mmap will
> fail with -EINVAL because of it.
This test has a initialization step in prepare() where it increases the
limit to at least a page if it is less than a page. Hence we'll never get
len = 0 here.

> In this case I think we can just skip the first mmap and only check that
> mmaping more than mlock limit fails.
> 
>>  	mem = mmap(NULL, len, prot, mode, fd, 0);
>>  	if (mem == MAP_FAILED) {
>>  		fail("unable to mmap secret memory\n");
>> -- 
>> 2.42.0
>>
> 

-- 
BR,
Muhammad Usama Anjum