bpf: add minimal BPF JIT emitted image disassembler

This is a minimal stand-alone user space helper, that allows for debugging or
verification of emitted BPF JIT images. This is in particular useful for
emitted opcode debugging, since minor bugs in the JIT compiler can be fatal.
The disassembler is architecture generic and uses libopcodes and libbfd.

How to get to the disassembly, example:

  1) `echo 2 > /proc/sys/net/core/bpf_jit_enable`
  2) Load a BPF filter (e.g. `tcpdump -p -n -s 0 -i eth1 host 192.168.20.0/24`)
  3) Run e.g. `bpf_jit_disasm -o` to disassemble the most recent JIT code output

`bpf_jit_disasm -o` will display the related opcodes to a particular instruction
as well. Example for x86_64:

$./bpf_jit_disasm
94 bytes emitted from JIT compiler (pass:3, flen:9)
ffffffffa0356000 + <x>:
   0:	push   %rbp
   1:	mov    %rsp,%rbp
   4:	sub    $0x60,%rsp
   8:	mov    %rbx,-0x8(%rbp)
   c:	mov    0x68(%rdi),%r9d
  10:	sub    0x6c(%rdi),%r9d
  14:	mov    0xe0(%rdi),%r8
  1b:	mov    $0xc,%esi
  20:	callq  0xffffffffe0d01b71
  25:	cmp    $0x86dd,%eax
  2a:	jne    0x000000000000003d
  2c:	mov    $0x14,%esi
  31:	callq  0xffffffffe0d01b8d
  36:	cmp    $0x6,%eax
[...]
  5c:	leaveq
  5d:	retq

$ ./bpf_jit_disasm -o
94 bytes emitted from JIT compiler (pass:3, flen:9)
ffffffffa0356000 + <x>:
   0:	push   %rbp
	55
   1:	mov    %rsp,%rbp
	48 89 e5
   4:	sub    $0x60,%rsp
	48 83 ec 60
   8:	mov    %rbx,-0x8(%rbp)
	48 89 5d f8
   c:	mov    0x68(%rdi),%r9d
	44 8b 4f 68
  10:	sub    0x6c(%rdi),%r9d
	44 2b 4f 6c
[...]
  5c:	leaveq
	c9
  5d:	retq
	c3

Note that this is also part of the Linux kernel source tree under
tools/net/. We also keep a copy of it here, since it is quite useful
in conjunction with bpfc for debugging purposes. However, we do not
integrate this small helper into the main build process. People who
care to have this need to compile it on their own which is not a big
deal.

Signed-off-by: Daniel Borkmann <dborkman@redhat.com>

1 files changed, 207 insertions, 0 deletions

diff --git a/bpf_jit_disasm.c b/bpf_jit_disasm.c
new file mode 100644
index 0000000..6d1df6e
--- /dev/null
+++ b/bpf_jit_disasm.c
@@ -0,0 +1,207 @@
+/*
+ * Minimal BPF JIT image disassembler
+ *
+ * Disassembles BPF JIT compiler emitted opcodes back to asm insn's for
+ * debugging or verification purposes.
+ *
+ * There is no Makefile. Compile with
+ *
+ *   `gcc -Wall -O2 bpf_jit_disasm.c -o bpf_jit_disasm -lopcodes -lbfd -ldl`
+ *
+ * or similar.
+ *
+ * To get the disassembly of the JIT code, do the following:
+ *
+ *  1) `echo 2 > /proc/sys/net/core/bpf_jit_enable`
+ *  2) Load a BPF filter (e.g. `tcpdump -p -n -s 0 -i eth1 host 192.168.20.0/24`)
+ *  3) Run e.g. `bpf_jit_disasm -o` to read out the last JIT code
+ *
+ * Copyright 2013 Daniel Borkmann <borkmann@redhat.com>
+ * Licensed under the GNU General Public License, version 2.0 (GPLv2)
+ *
+ * Note: this is part of the Linux kernel and can be found under tools/net/.
+ */
+
+#include <stdint.h>
+#include <stdio.h>
+#include <stdlib.h>
+#include <assert.h>
+#include <unistd.h>
+#include <string.h>
+#include <bfd.h>
+#include <dis-asm.h>
+#include <sys/klog.h>
+#include <sys/types.h>
+#include <regex.h>
+
+static void get_exec_path(char *tpath, size_t size)
+{
+	char *path;
+	ssize_t len;
+
+	snprintf(tpath, size, "/proc/%d/exe", (int) getpid());
+	tpath[size - 1] = 0;
+
+	path = strdup(tpath);
+	assert(path);
+
+	len = readlink(path, tpath, size);
+	tpath[len] = 0;
+
+	free(path);
+}
+
+static void get_asm_insns(uint8_t *image, size_t len, unsigned long base,
+			  int opcodes)
+{
+	int count, i, pc = 0;
+	char tpath[256];
+	struct disassemble_info info;
+	disassembler_ftype disassemble;
+	bfd *bfdf;
+
+	memset(tpath, 0, sizeof(tpath));
+	get_exec_path(tpath, sizeof(tpath));
+
+	bfdf = bfd_openr(tpath, NULL);
+	assert(bfdf);
+	assert(bfd_check_format(bfdf, bfd_object));
+
+	init_disassemble_info(&info, stdout, (fprintf_ftype) fprintf);
+	info.arch = bfd_get_arch(bfdf);
+	info.mach = bfd_get_mach(bfdf);
+	info.buffer = image;
+	info.buffer_length = len;
+
+	disassemble_init_for_target(&info);
+
+	disassemble = disassembler(bfdf);
+	assert(disassemble);
+
+	do {
+		printf("%4x:\t", pc);
+
+		count = disassemble(pc, &info);
+
+		if (opcodes) {
+			printf("\n\t");
+			for (i = 0; i < count; ++i)
+				printf("%02x ", (uint8_t) image[pc + i]);
+		}
+		printf("\n");
+
+		pc += count;
+	} while(count > 0 && pc < len);
+
+	bfd_close(bfdf);
+}
+
+static char *get_klog_buff(int *klen)
+{
+	int ret, len = klogctl(10, NULL, 0);
+	char *buff = malloc(len);
+
+	assert(buff && klen);
+	ret = klogctl(3, buff, len);
+	assert(ret >= 0);
+	*klen = ret;
+
+	return buff;
+}
+
+static void put_klog_buff(char *buff)
+{
+	free(buff);
+}
+
+static int get_last_jit_image(char *haystack, size_t hlen,
+			      uint8_t *image, size_t ilen,
+			      unsigned long *base)
+{
+	char *ptr, *pptr, *tmp;
+	off_t off = 0;
+	int ret, flen, proglen, pass, ulen = 0;
+	regmatch_t pmatch[1];
+	regex_t regex;
+
+	if (hlen == 0)
+		return 0;
+
+	ret = regcomp(&regex, "flen=[[:alnum:]]+ proglen=[[:digit:]]+ "
+		      "pass=[[:digit:]]+ image=[[:xdigit:]]+", REG_EXTENDED);
+	assert(ret == 0);
+
+	ptr = haystack;
+	while (1) {
+		ret = regexec(&regex, ptr, 1, pmatch, 0);
+		if (ret == 0) {
+			ptr += pmatch[0].rm_eo;
+			off += pmatch[0].rm_eo;
+			assert(off < hlen);
+		} else
+			break;
+	}
+
+	ptr = haystack + off - (pmatch[0].rm_eo - pmatch[0].rm_so);
+	ret = sscanf(ptr, "flen=%d proglen=%d pass=%d image=%lx",
+		     &flen, &proglen, &pass, base);
+	if (ret != 4)
+		return 0;
+
+	tmp = ptr = haystack + off;
+	while ((ptr = strtok(tmp, "\n")) != NULL && ulen < ilen) {
+		tmp = NULL;
+		if (!strstr(ptr, "JIT code"))
+			continue;
+		pptr = ptr;
+		while ((ptr = strstr(pptr, ":")))
+			pptr = ptr + 1;
+		ptr = pptr;
+		do {
+			image[ulen++] = (uint8_t) strtoul(pptr, &pptr, 16);
+			if (ptr == pptr || ulen >= ilen) {
+				ulen--;
+				break;
+			}
+			ptr = pptr;
+		} while (1);
+	}
+
+	assert(ulen == proglen);
+	printf("%d bytes emitted from JIT compiler (pass:%d, flen:%d)\n",
+	       proglen, pass, flen);
+	printf("%lx + <x>:\n", *base);
+
+	regfree(&regex);
+	return ulen;
+}
+
+int main(int argc, char **argv)
+{
+	int len, klen, opcodes = 0;
+	char *kbuff;
+	unsigned long base;
+	uint8_t image[4096];
+
+	if (argc > 1) {
+		if (!strncmp("-o", argv[argc - 1], 2)) {
+			opcodes = 1;
+		} else {
+			printf("usage: bpf_jit_disasm [-o: show opcodes]\n");
+			exit(0);
+		}
+	}
+
+	bfd_init();
+	memset(image, 0, sizeof(image));
+
+	kbuff = get_klog_buff(&klen);
+
+	len = get_last_jit_image(kbuff, klen, image, sizeof(image), &base);
+	if (len > 0 && base > 0)
+		get_asm_insns(image, len, base, opcodes);
+
+	put_klog_buff(kbuff);
+
+	return 0;
+}