Re: [PATCH v2 04/22] selftests/coredump: discard the right amount after the coredump request

[R Nageswara Sastry <rnsastry@linux.ibm.com>]

On 20.08.2026 4:39 AM, Christian Brauner wrote:
> read_coredump_req() gets the leftover wrong twice.
>
> It takes the absolute difference of the two sizes, so a test binary that
> knows a larger struct coredump_req than the kernel sends tries to discard
> bytes that were never sent. And it hands recv() sizeof(buffer) instead of
> the number of bytes it wants. So MSG_WAITALL waits for a whole page.
> Either one blocks until the kernel closes the socket. Which it won't
> because it is waiting for the coredump ack...
>
> It's benign today because struct coredump_req hasn't grown. But let's fix
> it for the future. Compute the leftover as what the kernel sent beyond
> what was consumed.
>
> Fixes: 59cd658eaf40 ("selftests/coredump: add coredump server selftests")
> Signed-off-by: Christian Brauner (Amutable) <brauner@kernel.org>
Tested-by: R Nageswara Sastry <rnsastry@linux.ibm.com>

System: ppc64le LPAR (IBM POWER), Linux 7.3-rc2
> ---
>   tools/testing/selftests/coredump/coredump_test_helpers.c | 8 ++++----
>   1 file changed, 4 insertions(+), 4 deletions(-)
>
> diff --git a/tools/testing/selftests/coredump/coredump_test_helpers.c b/tools/testing/selftests/coredump/coredump_test_helpers.c
> index 2a20faf9cb0a..524fa5370593 100644
> --- a/tools/testing/selftests/coredump/coredump_test_helpers.c
> +++ b/tools/testing/selftests/coredump/coredump_test_helpers.c
> @@ -235,10 +235,10 @@ bool read_coredump_req(int fd, struct coredump_req *req)
>   	fprintf(stderr, "Read coredump request with size %u and mask 0x%llx\n",
>   		req->size, (unsigned long long)req->mask);
>   
> -	if (user_size > kernel_size)
> -		remaining_size = user_size - kernel_size;
> -	else
> +	if (kernel_size > user_size)
>   		remaining_size = kernel_size - user_size;
> +	else
> +		remaining_size = 0;
>   
>   	if (PAGE_SIZE <= remaining_size)
>   		return false;
> @@ -250,7 +250,7 @@ bool read_coredump_req(int fd, struct coredump_req *req)
>   	if (remaining_size) {
>   		char buffer[PAGE_SIZE];
>   
> -		ret = recv(fd, buffer, sizeof(buffer), MSG_WAITALL);
> +		ret = recv(fd, buffer, remaining_size, MSG_WAITALL);
>   		if (ret != remaining_size)
>   			return false;
>   		fprintf(stderr, "Discarded %zu bytes of data after coredump request\n", remaining_size);
>
-- 
Thanks and Regards
R.Nageswara Sastry