[PATCH 10/11] coredump: describe the holes when COREDUMP_SPARSE is negotiated

[Christian Brauner <brauner@kernel.org>]

Offer COREDUMP_SPARSE in coredump_req->mask now that there is something
behind it. Refuse it without COREDUMP_HEADER. A zero frame cannot exist
outside a framed stream.

A trailing hole is flushed by coredump_write() the way it always was. So
its last byte goes out as a one-byte data frame and the frames cover the
whole coredump.

Signed-off-by: Christian Brauner (Amutable) <brauner@kernel.org>
---
 fs/coredump.c                                      | 34 +++++++++++++++++++++-
 .../selftests/coredump/coredump_test_helpers.c     |  3 +-
 2 files changed, 35 insertions(+), 2 deletions(-)

diff --git a/fs/coredump.c b/fs/coredump.c
index 364c89c5f82a..bdf2eae948d0 100644
--- a/fs/coredump.c
+++ b/fs/coredump.c
@@ -799,7 +799,7 @@ static bool coredump_sock_request(struct core_name *cn, struct coredump_params *
 		.size		= sizeof(struct coredump_req),
 		.mask		= COREDUMP_KERNEL | COREDUMP_USERSPACE |
 				  COREDUMP_REJECT | COREDUMP_WAIT |
-				  COREDUMP_HEADER,
+				  COREDUMP_HEADER | COREDUMP_SPARSE,
 		.size_ack	= sizeof(struct coredump_ack),
 	};
 	struct coredump_ack ack = {};
@@ -854,6 +854,12 @@ static bool coredump_sock_request(struct core_name *cn, struct coredump_params *
 		return false;
 	}
 
+	/* Zero frames only exist inside a framed stream. */
+	if ((ack.mask & COREDUMP_SPARSE) && !(ack.mask & COREDUMP_HEADER)) {
+		coredump_sock_mark(cprm->file, COREDUMP_MARK_CONFLICTING);
+		return false;
+	}
+
 	if (ack.spare) {
 		coredump_sock_mark(cprm->file, COREDUMP_MARK_UNSUPPORTED);
 		return false;
@@ -1229,6 +1235,11 @@ static bool dump_framed(const struct coredump_params *cprm)
 	return cprm->mask & COREDUMP_HEADER;
 }
 
+static bool dump_sparse(const struct coredump_params *cprm)
+{
+	return cprm->mask & COREDUMP_SPARSE;
+}
+
 /* Describe the next @len bytes of the coredump. Returns the header size. */
 static size_t dump_frame_init(struct coredump_params *cprm,
 			      enum coredump_frame_type type, u64 len)
@@ -1292,11 +1303,32 @@ static int __dump_emit(struct coredump_params *cprm, const void *addr, int nr)
 	return dump_write_iter(cprm, &iter, nr);
 }
 
+/* Hand the server the length of the hole instead of the hole itself. */
+static int dump_skip_frame(struct coredump_params *cprm, size_t nr)
+{
+	struct kvec kvec;
+	struct iov_iter iter;
+	size_t hdr;
+
+	if (dump_interrupted())
+		return 0;
+
+	hdr = dump_frame_init(cprm, COREDUMP_FRAME_ZERO, nr);
+	kvec.iov_base = cprm->frame;
+	kvec.iov_len = hdr;
+	iov_iter_kvec(&iter, ITER_SOURCE, &kvec, 1, hdr);
+
+	return dump_write_iter(cprm, &iter, nr);
+}
+
 static int __dump_skip(struct coredump_params *cprm, size_t nr)
 {
 	static char zeroes[PAGE_SIZE];
 	struct file *file = cprm->file;
 
+	if (dump_sparse(cprm))
+		return dump_skip_frame(cprm, nr);
+
 	if (file->f_mode & FMODE_LSEEK) {
 		if (dump_interrupted() || vfs_llseek(file, nr, SEEK_CUR) < 0)
 			return 0;
diff --git a/tools/testing/selftests/coredump/coredump_test_helpers.c b/tools/testing/selftests/coredump/coredump_test_helpers.c
index bfe10bc51618..ee669969605b 100644
--- a/tools/testing/selftests/coredump/coredump_test_helpers.c
+++ b/tools/testing/selftests/coredump/coredump_test_helpers.c
@@ -290,7 +290,8 @@ bool send_coredump_ack(int fd, const struct coredump_req *req,
 /* Every option the kernel is expected to advertise in coredump_req->mask. */
 #define COREDUMP_REQ_MASK_ALL					\
 	(COREDUMP_KERNEL | COREDUMP_USERSPACE |			\
-	 COREDUMP_REJECT | COREDUMP_WAIT | COREDUMP_HEADER)
+	 COREDUMP_REJECT | COREDUMP_WAIT |			\
+	 COREDUMP_HEADER | COREDUMP_SPARSE)
 
 bool check_coredump_req(const struct coredump_req *req)
 {

-- 
2.53.0