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 <[email protected]>
> ---
> 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