From c231c22a989af95fae3e75cac9d4511e0fe79377 Mon Sep 17 00:00:00 2001 From: Jiong Wang Date: Thu, 30 May 2019 21:23:18 +0100 Subject: bpf: doc: update answer for 32-bit subregister question There has been quite a few progress around the two steps mentioned in the answer to the following question: Q: BPF 32-bit subregister requirements This patch updates the answer to reflect what has been done. v2: - Add missing full stop. (Song Liu) - Minor tweak on one sentence. (Song Liu) v1: - Integrated rephrase from Quentin and Jakub Reviewed-by: Quentin Monnet Reviewed-by: Jakub Kicinski Signed-off-by: Jiong Wang Acked-by: Song Liu Signed-off-by: Alexei Starovoitov --- Documentation/bpf/bpf_design_QA.rst | 30 +++++++++++++++++++++++++----- 1 file changed, 25 insertions(+), 5 deletions(-) (limited to 'Documentation/bpf') diff --git a/Documentation/bpf/bpf_design_QA.rst b/Documentation/bpf/bpf_design_QA.rst index cb402c59eca5..12a246fcf6cb 100644 --- a/Documentation/bpf/bpf_design_QA.rst +++ b/Documentation/bpf/bpf_design_QA.rst @@ -172,11 +172,31 @@ registers which makes BPF inefficient virtual machine for 32-bit CPU architectures and 32-bit HW accelerators. Can true 32-bit registers be added to BPF in the future? -A: NO. The first thing to improve performance on 32-bit archs is to teach -LLVM to generate code that uses 32-bit subregisters. Then second step -is to teach verifier to mark operations where zero-ing upper bits -is unnecessary. Then JITs can take advantage of those markings and -drastically reduce size of generated code and improve performance. +A: NO. + +But some optimizations on zero-ing the upper 32 bits for BPF registers are +available, and can be leveraged to improve the performance of JITed BPF +programs for 32-bit architectures. + +Starting with version 7, LLVM is able to generate instructions that operate +on 32-bit subregisters, provided the option -mattr=+alu32 is passed for +compiling a program. Furthermore, the verifier can now mark the +instructions for which zero-ing the upper bits of the destination register +is required, and insert an explicit zero-extension (zext) instruction +(a mov32 variant). This means that for architectures without zext hardware +support, the JIT back-ends do not need to clear the upper bits for +subregisters written by alu32 instructions or narrow loads. Instead, the +back-ends simply need to support code generation for that mov32 variant, +and to overwrite bpf_jit_needs_zext() to make it return "true" (in order to +enable zext insertion in the verifier). + +Note that it is possible for a JIT back-end to have partial hardware +support for zext. In that case, if verifier zext insertion is enabled, +it could lead to the insertion of unnecessary zext instructions. Such +instructions could be removed by creating a simple peephole inside the JIT +back-end: if one instruction has hardware support for zext and if the next +instruction is an explicit zext, then the latter can be skipped when doing +the code generation. Q: Does BPF have a stable ABI? ------------------------------ -- cgit v1.2.3 From 0c51b3697a446e904e9e9fb70bbd7d08606a329a Mon Sep 17 00:00:00 2001 From: Stanislav Fomichev Date: Thu, 27 Jun 2019 13:38:54 -0700 Subject: bpf: add sockopt documentation Provide user documentation about sockopt prog type and cgroup hooks. v9: * add details about setsockopt context and inheritance v7: * add description for retval=0 and optlen=-1 v6: * describe cgroup chaining, add example v2: * use return code 2 for kernel bypass Cc: Andrii Nakryiko Cc: Martin Lau Signed-off-by: Stanislav Fomichev Signed-off-by: Alexei Starovoitov --- Documentation/bpf/index.rst | 1 + Documentation/bpf/prog_cgroup_sockopt.rst | 93 +++++++++++++++++++++++++++++++ 2 files changed, 94 insertions(+) create mode 100644 Documentation/bpf/prog_cgroup_sockopt.rst (limited to 'Documentation/bpf') diff --git a/Documentation/bpf/index.rst b/Documentation/bpf/index.rst index d3fe4cac0c90..801a6ed3f2e5 100644 --- a/Documentation/bpf/index.rst +++ b/Documentation/bpf/index.rst @@ -42,6 +42,7 @@ Program types .. toctree:: :maxdepth: 1 + prog_cgroup_sockopt prog_cgroup_sysctl prog_flow_dissector diff --git a/Documentation/bpf/prog_cgroup_sockopt.rst b/Documentation/bpf/prog_cgroup_sockopt.rst new file mode 100644 index 000000000000..c47d974629ae --- /dev/null +++ b/Documentation/bpf/prog_cgroup_sockopt.rst @@ -0,0 +1,93 @@ +.. SPDX-License-Identifier: GPL-2.0 + +============================ +BPF_PROG_TYPE_CGROUP_SOCKOPT +============================ + +``BPF_PROG_TYPE_CGROUP_SOCKOPT`` program type can be attached to two +cgroup hooks: + +* ``BPF_CGROUP_GETSOCKOPT`` - called every time process executes ``getsockopt`` + system call. +* ``BPF_CGROUP_SETSOCKOPT`` - called every time process executes ``setsockopt`` + system call. + +The context (``struct bpf_sockopt``) has associated socket (``sk``) and +all input arguments: ``level``, ``optname``, ``optval`` and ``optlen``. + +BPF_CGROUP_SETSOCKOPT +===================== + +``BPF_CGROUP_SETSOCKOPT`` is triggered *before* the kernel handling of +sockopt and it has writable context: it can modify the supplied arguments +before passing them down to the kernel. This hook has access to the cgroup +and socket local storage. + +If BPF program sets ``optlen`` to -1, the control will be returned +back to the userspace after all other BPF programs in the cgroup +chain finish (i.e. kernel ``setsockopt`` handling will *not* be executed). + +Note, that ``optlen`` can not be increased beyond the user-supplied +value. It can only be decreased or set to -1. Any other value will +trigger ``EFAULT``. + +Return Type +----------- + +* ``0`` - reject the syscall, ``EPERM`` will be returned to the userspace. +* ``1`` - success, continue with next BPF program in the cgroup chain. + +BPF_CGROUP_GETSOCKOPT +===================== + +``BPF_CGROUP_GETSOCKOPT`` is triggered *after* the kernel handing of +sockopt. The BPF hook can observe ``optval``, ``optlen`` and ``retval`` +if it's interested in whatever kernel has returned. BPF hook can override +the values above, adjust ``optlen`` and reset ``retval`` to 0. If ``optlen`` +has been increased above initial ``getsockopt`` value (i.e. userspace +buffer is too small), ``EFAULT`` is returned. + +This hook has access to the cgroup and socket local storage. + +Note, that the only acceptable value to set to ``retval`` is 0 and the +original value that the kernel returned. Any other value will trigger +``EFAULT``. + +Return Type +----------- + +* ``0`` - reject the syscall, ``EPERM`` will be returned to the userspace. +* ``1`` - success: copy ``optval`` and ``optlen`` to userspace, return + ``retval`` from the syscall (note that this can be overwritten by + the BPF program from the parent cgroup). + +Cgroup Inheritance +================== + +Suppose, there is the following cgroup hierarchy where each cgroup +has ``BPF_CGROUP_GETSOCKOPT`` attached at each level with +``BPF_F_ALLOW_MULTI`` flag:: + + A (root, parent) + \ + B (child) + +When the application calls ``getsockopt`` syscall from the cgroup B, +the programs are executed from the bottom up: B, A. First program +(B) sees the result of kernel's ``getsockopt``. It can optionally +adjust ``optval``, ``optlen`` and reset ``retval`` to 0. After that +control will be passed to the second (A) program which will see the +same context as B including any potential modifications. + +Same for ``BPF_CGROUP_SETSOCKOPT``: if the program is attached to +A and B, the trigger order is B, then A. If B does any changes +to the input arguments (``level``, ``optname``, ``optval``, ``optlen``), +then the next program in the chain (A) will see those changes, +*not* the original input ``setsockopt`` arguments. The potentially +modified values will be then passed down to the kernel. + +Example +======= + +See ``tools/testing/selftests/bpf/progs/sockopt_sk.c`` for an example +of BPF program that handles socket options. -- cgit v1.2.3