Add an example in map documentation to show how to use bpftool in order
to update the references to programs hold by prog array maps.
Signed-off-by: Quentin Monnet <quentin.mon...@netronome.com>
Reviewed-by: Jakub Kicinski <jakub.kicin...@netronome.com>
---
tools/bpf/bpftool/Documentation/bpftool-map.rst | 55 +++++++++++++++++++++++++
1 file changed, 55 insertions(+)
diff --git a/tools/bpf/bpftool/Documentation/bpftool-map.rst
b/tools/bpf/bpftool/Documentation/bpftool-map.rst
index b73d39af7b32..64b001b4f777 100644
--- a/tools/bpf/bpftool/Documentation/bpftool-map.rst
+++ b/tools/bpf/bpftool/Documentation/bpftool-map.rst
@@ -174,6 +174,61 @@ The following three commands are equivalent:
| **# bpftool map pin id 10 /sys/fs/bpf/map**
| **# bpftool map del pinned /sys/fs/bpf/map key 13 00 07 00**
+Note that map update can also be used in order to change the program references
+hold by a program array map. This can be used, for example, to change the
+programs used for tail-call jumps at runtime, without having to reload the
+entry-point program. Below is an example for this use case: we load a program
+defining a prog array map, and with a main function that contains a tail call
+to other programs that can be used either to "process" packets or to "debug"
+processing. Note that the prog array map MUST be pinned into the BPF virtual
+file system for the map update to work successfully, as kernel flushes prog
+array maps when they have no more references from user space (and the update
+would be lost as soon as bpftool exits).
+
+|
+| **# bpftool prog loadall tail_calls.o /sys/fs/bpf/foo type xdp**
+| **# bpftool prog --bpffs**
+
+::
+
+ 545: xdp name main_func tag 674b4b5597193dc3 gpl
+ loaded_at 2018-12-12T15:02:58+0000 uid 0
+ xlated 240B jited 257B memlock 4096B map_ids 294
+ pinned /sys/fs/bpf/foo/xdp
+ 546: xdp name bpf_func_process tag e369a529024751fc gpl
+ loaded_at 2018-12-12T15:02:58+0000 uid 0
+ xlated 200B jited 164B memlock 4096B
+ pinned /sys/fs/bpf/foo/process
+ 547: xdp name bpf_func_debug tag 0b597868bc7f0976 gpl
+ loaded_at 2018-12-12T15:02:58+0000 uid 0
+ xlated 200B jited 164B memlock 4096B
+ pinned /sys/fs/bpf/foo/debug
+
+**# bpftool map**
+
+::
+
+ 294: prog_array name jmp_table flags 0x0
+ key 4B value 4B max_entries 1 memlock 4096B
+ owner_prog_type xdp owner jited
+
+|
+| **# bpftool map pin id 294 /sys/fs/bpf/bar**
+| **# bpftool map dump pinned /sys/fs/bpf/bar**
+
+::
+
+ Found 0 elements
+
+|
+| **# bpftool map update pinned /sys/fs/bpf/bar key 0 0 0 0 value pinned
/sys/fs/bpf/foo/debug**
+| **# bpftool map dump pinned /sys/fs/bpf/bar**
+
+::
+
+ key: 00 00 00 00 value: 22 02 00 00
+ Found 1 element
+
SEE ALSO
========
**bpf**\ (2),
--
2.7.4
