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authorDave Thaler <dthaler@microsoft.com>2023-03-08 23:53:03 +0300
committerAlexei Starovoitov <ast@kernel.org>2023-03-11 00:02:00 +0300
commitc1f9e14e3b676eb88fe1c9488c0b5f4fc9108a1c (patch)
treec895db124ca1ed63b2683c3136c3a83f2fd7de0a /Documentation/bpf
parent74843b57ec70af7b67b7e6153374834ee18d139f (diff)
downloadlinux-c1f9e14e3b676eb88fe1c9488c0b5f4fc9108a1c.tar.xz
bpf, docs: Explain helper functions
Add brief text about existence of helper functions, with details to go in separate psABI text. Note that text about runtime functions (kfuncs) is part of a separate patch, not this one. Signed-off-by: Dave Thaler <dthaler@microsoft.com> Link: https://lore.kernel.org/r/20230308205303.1308-1-dthaler1968@googlemail.com Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Diffstat (limited to 'Documentation/bpf')
-rw-r--r--Documentation/bpf/clang-notes.rst6
-rw-r--r--Documentation/bpf/instruction-set.rst9
-rw-r--r--Documentation/bpf/linux-notes.rst8
3 files changed, 22 insertions, 1 deletions
diff --git a/Documentation/bpf/clang-notes.rst b/Documentation/bpf/clang-notes.rst
index 528feddf2db9..2c872a1ee08e 100644
--- a/Documentation/bpf/clang-notes.rst
+++ b/Documentation/bpf/clang-notes.rst
@@ -20,6 +20,12 @@ Arithmetic instructions
For CPU versions prior to 3, Clang v7.0 and later can enable ``BPF_ALU`` support with
``-Xclang -target-feature -Xclang +alu32``. In CPU version 3, support is automatically included.
+Jump instructions
+=================
+
+If ``-O0`` is used, Clang will generate the ``BPF_CALL | BPF_X | BPF_JMP`` (0x8d)
+instruction, which is not supported by the Linux kernel verifier.
+
Atomic operations
=================
diff --git a/Documentation/bpf/instruction-set.rst b/Documentation/bpf/instruction-set.rst
index db8789e6969e..5e43e14abe80 100644
--- a/Documentation/bpf/instruction-set.rst
+++ b/Documentation/bpf/instruction-set.rst
@@ -253,7 +253,7 @@ BPF_JSET 0x40 PC += off if dst & src
BPF_JNE 0x50 PC += off if dst != src
BPF_JSGT 0x60 PC += off if dst > src signed
BPF_JSGE 0x70 PC += off if dst >= src signed
-BPF_CALL 0x80 function call
+BPF_CALL 0x80 function call see `Helper functions`_
BPF_EXIT 0x90 function / program return BPF_JMP only
BPF_JLT 0xa0 PC += off if dst < src unsigned
BPF_JLE 0xb0 PC += off if dst <= src unsigned
@@ -264,6 +264,13 @@ BPF_JSLE 0xd0 PC += off if dst <= src signed
The eBPF program needs to store the return value into register R0 before doing a
BPF_EXIT.
+Helper functions
+~~~~~~~~~~~~~~~~
+
+Helper functions are a concept whereby BPF programs can call into a
+set of function calls exposed by the runtime. Each helper
+function is identified by an integer used in a ``BPF_CALL`` instruction.
+The available helper functions may differ for each program type.
Load and store instructions
===========================
diff --git a/Documentation/bpf/linux-notes.rst b/Documentation/bpf/linux-notes.rst
index 956b0c86699d..f43b9c797bcb 100644
--- a/Documentation/bpf/linux-notes.rst
+++ b/Documentation/bpf/linux-notes.rst
@@ -12,6 +12,14 @@ Byte swap instructions
``BPF_FROM_LE`` and ``BPF_FROM_BE`` exist as aliases for ``BPF_TO_LE`` and ``BPF_TO_BE`` respectively.
+Jump instructions
+=================
+
+``BPF_CALL | BPF_X | BPF_JMP`` (0x8d), where the helper function
+integer would be read from a specified register, is not currently supported
+by the verifier. Any programs with this instruction will fail to load
+until such support is added.
+
Legacy BPF Packet access instructions
=====================================