sys/linux: describe map holding constant string

This goal of this commit is to prepare a map whose value will be of type
ARG_PTR_TO_CONST_STR when seen by the BPF verifier [1]. To that end, the
map must be read-only, of a type that allows direct map value access
(only array maps), and frozen. In addition, it must contains a
zero-terminated string as its value.

This commit therefore defines a new bpf$MAP_CREATE_CONST_STR syscall
description to create a read-only array map of fixed size. A new
bpf$MAP_UPDATE_CONST_STR syscall description then updates the map with
a zero-terminated string. Finally, bpf$BPF_MAP_CONST_STR_FREEZE freezes
the map to prevent any other updates from the syscall side (BPF side
updates are already prevented since the map is read-only).

As a result, we want to end up with an fd to a map that has been
created, updated, and frozen. To guarantee that all operations have been
carried out (in the correct order) on the map, we change the map fd
after each operation with the following example syntax:

    map_bpf_const_str {
        in     fd_bpf_const_str_map
        out    fd_bpf_const_str        (out_overlay)
    }

The 'in' fd is passed on entry to bpf$MAP_UPDATE_CONST_STR and the 'out'
fd is returned. In practice, the fd value will not be changed, but this
description allows us to reference the fd_bpf_const_str type in
subsequent operations, thus ensuring we're using an fd to an updated
map.

1 - https://github.com/torvalds/linux/blob/8a749fd1a8720d4619c91c8b6e7528c0a355c0aa/kernel/bpf/verifier.c#L8334
Signed-off-by: Paul Chaignon <paul.chaignon@gmail.com>

1 files changed, 26 insertions, 0 deletions

diff --git a/sys/linux/bpf.txt b/sys/linux/bpf.txt
index 52a6575cd..b50f49ef2 100644
--- a/sys/linux/bpf.txt
+++ b/sys/linux/bpf.txt
@@ -19,6 +19,9 @@ resource bpf_btf_id[int32]: 0, -1
 resource bpf_link_id[int32]: 0, -1
 resource fd_bpf_link[fd]
 resource bpf_revision[int64]
+resource fd_bpf_const_str_map[fd_bpf_map]
+resource fd_bpf_const_str[fd_bpf_const_str_map]
+resource bpf_frozen_const_str[fd_bpf_const_str]
 
 # NEED: this is a random index in btf_header:types. We can't express this, so we just use a small index.
 type btf_type_id int32[1:5]
@@ -33,8 +36,10 @@ type btf_opt_name_off int32[0:16]
 type map_fd_id int32[0:16]
 
 bpf$MAP_CREATE(cmd const[BPF_MAP_CREATE], arg ptr[in, bpf_map_create_arg], size len[arg]) fd_bpf_map
+bpf$MAP_CREATE_CONST_STR(cmd const[BPF_MAP_CREATE], arg ptr[in, bpf_map_create_arg_const_str], size len[arg]) fd_bpf_const_str_map
 bpf$MAP_LOOKUP_ELEM(cmd const[BPF_MAP_LOOKUP_ELEM], arg ptr[in, bpf_map_lookup_arg], size len[arg])
 bpf$MAP_UPDATE_ELEM(cmd const[BPF_MAP_UPDATE_ELEM], arg ptr[in, bpf_map_update_arg], size len[arg])
+bpf$MAP_UPDATE_CONST_STR(cmd const[BPF_MAP_UPDATE_ELEM], arg ptr[inout, bpf_map_update_const_str_arg], size len[arg])
 bpf$MAP_DELETE_ELEM(cmd const[BPF_MAP_DELETE_ELEM], arg ptr[in, bpf_map_delete_arg], size len[arg])
 bpf$MAP_GET_NEXT_KEY(cmd const[BPF_MAP_GET_NEXT_KEY], arg ptr[in, bpf_map_get_next_arg], size len[arg])
 bpf$PROG_LOAD(cmd const[BPF_PROG_LOAD], arg ptr[in, bpf_prog], size len[arg]) fd_bpf_prog
@@ -59,6 +64,7 @@ bpf$BPF_BTF_GET_FD_BY_ID(cmd const[BPF_BTF_GET_FD_BY_ID], arg ptr[in, bpf_btf_id
 bpf$BPF_TASK_FD_QUERY(cmd const[BPF_TASK_FD_QUERY], arg ptr[inout, bpf_task_fd_query], size len[arg])
 bpf$BPF_MAP_LOOKUP_AND_DELETE_ELEM(cmd const[BPF_MAP_LOOKUP_AND_DELETE_ELEM], arg ptr[in, bpf_map_lookup_arg], size len[arg])
 bpf$BPF_MAP_FREEZE(cmd const[BPF_MAP_FREEZE], arg ptr[in, fd_bpf_map], size len[arg])
+bpf$BPF_MAP_CONST_STR_FREEZE(cmd const[BPF_MAP_FREEZE], arg ptr[inout, bpf_map_const_str_freeze], size len[arg])
 bpf$MAP_LOOKUP_BATCH(cmd const[BPF_MAP_LOOKUP_BATCH], arg ptr[in, bpf_map_batch_arg], size len[arg])
 bpf$MAP_UPDATE_BATCH(cmd const[BPF_MAP_UPDATE_BATCH], arg ptr[in, bpf_map_batch_arg], size len[arg])
 bpf$MAP_DELETE_BATCH(cmd const[BPF_MAP_DELETE_BATCH], arg ptr[in, bpf_map_batch_arg], size len[arg])
@@ -80,6 +86,11 @@ type bpf_prog_xdp bpf_prog_t[const[BPF_PROG_TYPE_XDP, int32], const[BPF_XDP, int
 type bpf_link_create_xdp bpf_link_create_arg_t[fd_bpf_prog_xdp, ifindex, const[BPF_XDP, int32], flags[xdp_flags, int32]]
 xdp_flags = XDP_FLAGS_UPDATE_IF_NOEXIST, XDP_FLAGS_SKB_MODE, XDP_FLAGS_DRV_MODE, XDP_FLAGS_HW_MODE, XDP_FLAGS_REPLACE
 
+bpf_map_const_str_freeze {
+	in	fd_bpf_const_str
+	out	bpf_frozen_const_str	(out_overlay)
+}
+
 bpf_map_create_arg [
 	base		bpf_map_create_arg_base
 	bloom_filter	bpf_map_create_arg_bf
@@ -104,6 +115,7 @@ type bpf_map_create_arg_t[TYPE, KSIZE, VSIZE, MAX, FLAGS, MAP_EXTRA] {
 
 type bpf_map_create_arg_base bpf_map_create_arg_t[flags[bpf_map_type, int32], int32, int32, int32, flags[map_flags, int32], const[0, int64]]
 type bpf_map_create_arg_bf bpf_map_create_arg_t[const[BPF_MAP_TYPE_BLOOM_FILTER, int32], int32, int32, int32, flags[map_flags, int32], int64[0:15]]
+type bpf_map_create_arg_const_str bpf_map_create_arg_t[const[BPF_MAP_TYPE_ARRAY, int32], const[4, int32], const[8, int32], const[1, int32], const[BPF_F_RDONLY_PROG, int32], const[0, int64]]
 
 bpf_map_get_fd_by_id_arg {
 	map_id		bpf_map_id
@@ -133,6 +145,20 @@ bpf_map_update_arg {
 	flags	flags[bpf_map_flags, int64]
 }
 
+map_bpf_const_str {
+	in	fd_bpf_const_str_map
+	out	fd_bpf_const_str	(out_overlay)
+}
+
+bpf_printf_str = "%d     ", "%p     "
+
+bpf_map_update_const_str_arg {
+	map	map_bpf_const_str
+	key	ptr64[in, const[0, int32]]
+	val	ptr64[in, string[bpf_printf_str]]
+	flags	const[0, int64]
+}
+
 bpf_map_delete_arg {
 	map	fd_bpf_map
 	key	ptr64[in, array[int8]]