From 8ac270d1e29f0428228ab2b9a8ae5e1ed4a5cd84 Mon Sep 17 00:00:00 2001 From: Will Drewry Date: Thu, 12 Apr 2012 16:48:04 -0500 Subject: Documentation: prctl/seccomp_filter Documents how system call filtering using Berkeley Packet Filter programs works and how it may be used. Includes an example for x86 and a semi-generic example using a macro-based code generator. Acked-by: Eric Paris Signed-off-by: Will Drewry Acked-by: Kees Cook v18: - added acked by - update no new privs numbers v17: - remove @compat note and add Pitfalls section for arch checking (keescook@chromium.org) v16: - v15: - v14: - rebase/nochanges v13: - rebase on to 88ebdda6159ffc15699f204c33feb3e431bf9bdc v12: - comment on the ptrace_event use - update arch support comment - note the behavior of SECCOMP_RET_DATA when there are multiple filters (keescook@chromium.org) - lots of samples/ clean up incl 64-bit bpf-direct support (markus@chromium.org) - rebase to linux-next v11: - overhaul return value language, updates (keescook@chromium.org) - comment on do_exit(SIGSYS) v10: - update for SIGSYS - update for new seccomp_data layout - update for ptrace option use v9: - updated bpf-direct.c for SIGILL v8: - add PR_SET_NO_NEW_PRIVS to the samples. v7: - updated for all the new stuff in v7: TRAP, TRACE - only talk about PR_SET_SECCOMP now - fixed bad JLE32 check (coreyb@linux.vnet.ibm.com) - adds dropper.c: a simple system call disabler v6: - tweak the language to note the requirement of PR_SET_NO_NEW_PRIVS being called prior to use. (luto@mit.edu) v5: - update sample to use system call arguments - adds a "fancy" example using a macro-based generator - cleaned up bpf in the sample - update docs to mention arguments - fix prctl value (eparis@redhat.com) - language cleanup (rdunlap@xenotime.net) v4: - update for no_new_privs use - minor tweaks v3: - call out BPF <-> Berkeley Packet Filter (rdunlap@xenotime.net) - document use of tentative always-unprivileged - guard sample compilation for i386 and x86_64 v2: - move code to samples (corbet@lwn.net) Signed-off-by: James Morris --- samples/seccomp/bpf-helper.c | 89 ++++++++++++++++++++++++++++++++++++++++++++ 1 file changed, 89 insertions(+) create mode 100644 samples/seccomp/bpf-helper.c (limited to 'samples/seccomp/bpf-helper.c') diff --git a/samples/seccomp/bpf-helper.c b/samples/seccomp/bpf-helper.c new file mode 100644 index 0000000..579cfe3 --- /dev/null +++ b/samples/seccomp/bpf-helper.c @@ -0,0 +1,89 @@ +/* + * Seccomp BPF helper functions + * + * Copyright (c) 2012 The Chromium OS Authors + * Author: Will Drewry + * + * The code may be used by anyone for any purpose, + * and can serve as a starting point for developing + * applications using prctl(PR_ATTACH_SECCOMP_FILTER). + */ + +#include +#include + +#include "bpf-helper.h" + +int bpf_resolve_jumps(struct bpf_labels *labels, + struct sock_filter *filter, size_t count) +{ + struct sock_filter *begin = filter; + __u8 insn = count - 1; + + if (count < 1) + return -1; + /* + * Walk it once, backwards, to build the label table and do fixups. + * Since backward jumps are disallowed by BPF, this is easy. + */ + filter += insn; + for (; filter >= begin; --insn, --filter) { + if (filter->code != (BPF_JMP+BPF_JA)) + continue; + switch ((filter->jt<<8)|filter->jf) { + case (JUMP_JT<<8)|JUMP_JF: + if (labels->labels[filter->k].location == 0xffffffff) { + fprintf(stderr, "Unresolved label: '%s'\n", + labels->labels[filter->k].label); + return 1; + } + filter->k = labels->labels[filter->k].location - + (insn + 1); + filter->jt = 0; + filter->jf = 0; + continue; + case (LABEL_JT<<8)|LABEL_JF: + if (labels->labels[filter->k].location != 0xffffffff) { + fprintf(stderr, "Duplicate label use: '%s'\n", + labels->labels[filter->k].label); + return 1; + } + labels->labels[filter->k].location = insn; + filter->k = 0; /* fall through */ + filter->jt = 0; + filter->jf = 0; + continue; + } + } + return 0; +} + +/* Simple lookup table for labels. */ +__u32 seccomp_bpf_label(struct bpf_labels *labels, const char *label) +{ + struct __bpf_label *begin = labels->labels, *end; + int id; + if (labels->count == 0) { + begin->label = label; + begin->location = 0xffffffff; + labels->count++; + return 0; + } + end = begin + labels->count; + for (id = 0; begin < end; ++begin, ++id) { + if (!strcmp(label, begin->label)) + return id; + } + begin->label = label; + begin->location = 0xffffffff; + labels->count++; + return id; +} + +void seccomp_bpf_print(struct sock_filter *filter, size_t count) +{ + struct sock_filter *end = filter + count; + for ( ; filter < end; ++filter) + printf("{ code=%u,jt=%u,jf=%u,k=%u },\n", + filter->code, filter->jt, filter->jf, filter->k); +} -- cgit v1.1