A coredump with a lot of unpopulated mappings sends endless amounts of zero data to userspace. This is nonsensical. While __dump_skip() can seek over them when the target is a regular file a socket cannot do this. COREDUMP_HEADER framed the zeroes but it didn't get rid of them.
Add a COREDUMP_SPARSE feature bit and a COREDUMP_FRAME_ZERO frame type. A zero frame is a bare header that tells userspace how many zero bytes where skipped. So a hole crosses the socket as one header no matter how long it is. The coredump server can recreate this sparsely. Zero frames only exist inside a framed stream. So COREDUMP_SPARSE requires COREDUMP_HEADER. Signed-off-by: Christian Brauner (Amutable) <[email protected]> --- include/uapi/linux/coredump.h | 15 ++++++++++++--- 1 file changed, 12 insertions(+), 3 deletions(-) diff --git a/include/uapi/linux/coredump.h b/include/uapi/linux/coredump.h index 5252480d3eec..312bafabb467 100644 --- a/include/uapi/linux/coredump.h +++ b/include/uapi/linux/coredump.h @@ -14,6 +14,8 @@ * @COREDUMP_HEADER: send the coredump as a sequence of frames instead of * as a plain byte stream, see struct coredump_frame_header; * requires COREDUMP_KERNEL + * @COREDUMP_SPARSE: describe the holes in the coredump as zero frames + * instead of transferring them; requires COREDUMP_HEADER */ enum { COREDUMP_KERNEL = (1ULL << 0), @@ -21,6 +23,7 @@ enum { COREDUMP_REJECT = (1ULL << 2), COREDUMP_WAIT = (1ULL << 3), COREDUMP_HEADER = (1ULL << 4), + COREDUMP_SPARSE = (1ULL << 5), }; /** @@ -109,10 +112,13 @@ enum coredump_mark { * enum coredump_frame_type - Type of a coredump frame * * @COREDUMP_FRAME_DATA: the header is followed by ->len bytes of data + * @COREDUMP_FRAME_ZERO: the header stands for ->len zero bytes and is not + * followed by any data * @__COREDUMP_FRAME_MAX: the maximum coredump frame type value */ enum coredump_frame_type { COREDUMP_FRAME_DATA = 0U, + COREDUMP_FRAME_ZERO = 1U, __COREDUMP_FRAME_MAX = (1U << 31), }; @@ -126,8 +132,10 @@ enum coredump_frame_type { * * If the coredump server raises COREDUMP_HEADER in coredump_ack->mask the * kernel doesn't send the coredump as a plain byte stream. It sends a - * sequence of frames instead. A struct coredump_frame_header is followed by - * @len bytes of actual coredump data. + * sequence of frames instead. A COREDUMP_FRAME_DATA frame is followed by + * @len bytes of actual coredump data. A COREDUMP_FRAME_ZERO frame is + * followed by nothing and stands for @len zero bytes. A server that didn't + * raise COREDUMP_SPARSE never sees a zero frame. * * The @size member is set to the size of struct coredump_frame_header the * kernel knows and lets the header grow later. It comes first so it can be @@ -139,7 +147,8 @@ enum coredump_frame_type { * interpreted. No flags are defined yet. Userspace must refuse a frame * carrying a flag it doesn't know. * - * COREDUMP_HEADER must be combined with COREDUMP_KERNEL. + * COREDUMP_HEADER must be combined with COREDUMP_KERNEL, and + * COREDUMP_SPARSE with COREDUMP_HEADER. */ struct coredump_frame_header { __u32 size; -- 2.53.0
