Em Mon, Nov 13, 2017 at 03:56:14PM +0100, Daniel Borkmann escreveu:

On 11/13/2017 03:30 PM, Arnaldo Carvalho de Melo wrote:

quoted

Hi,

	In a5e8c07059d0 ("bpf: add bpf_probe_read_str helper") you
state:

   "This is suboptimal because the size of the string needs to be estimated
    at compile time, causing more memory to be copied than often necessary,
    and can become more problematic if further processing on buf is done,
    for example by pushing it to userspace via bpf_perf_event_output(),
    since the real length of the string is unknown and the entire buffer
    must be copied (and defining an unrolled strnlen() inside the bpf
    program is a very inefficient and unfeasible approach)."

So I went on to try this with 'perf trace' but it isn't working if I use
the return from bpf_probe_read_str(), I must be missing something
here... 

I.e. this works:

[root@jouet bpf]# cat open.c
#include "bpf.h"

SEC("prog=do_sys_open filename")
int prog(void *ctx, int err, const char __user *filename_ptr)
{
	char filename[128];
	const unsigned len = bpf_probe_read_str(filename, sizeof(filename), filename_ptr);
	perf_event_output(ctx, &__bpf_stdout__, get_smp_processor_id(), filename, 32);

By the way, you can just use BPF_F_CURRENT_CPU flag instead of the helper
call get_smp_processor_id() to get current CPU.

Thanks, switched to it.

quoted

But then if I use the return value to push just the string lenght, it
doesn't work:

[root@jouet bpf]# cat open.c
#include "bpf.h"

SEC("prog=do_sys_open filename")
int prog(void *ctx, int err, const char __user *filename_ptr)
{
	char filename[128];
	const unsigned len = bpf_probe_read_str(filename, sizeof(filename), filename_ptr);
	perf_event_output(ctx, &__bpf_stdout__, get_smp_processor_id(), filename, len);

The below issue 'invalid stack type R4 off=-128 access_size=0' is basically that
unsigned len is unknown at verification time, thus unbounded. Can you try the
following to see if that passes?

if (len > 0 && len <= sizeof(filename))
    perf_event_output(ctx, &__bpf_stdout__, get_smp_processor_id(), filename, len);

I had it like:

	if (len > 0 && len < 32)

And it didn't helped, now I did exactly as you suggested:

[root@jouet bpf]# cat open.c
#include "bpf.h"

SEC("prog=do_sys_open filename")
int prog(void *ctx, int err, const char __user *filename_ptr)
{
	char filename[128];
	const unsigned len = bpf_probe_read_str(filename, sizeof(filename), filename_ptr);
	if (len > 0 && len <= sizeof(filename))
		perf_event_output(ctx, &__bpf_stdout__, BPF_F_CURRENT_CPU, filename, len);
	return 1;
}
[root@jouet bpf]# trace -e open,open.c touch /etc/passwd
bpf: builtin compilation failed: -95, try external compiler
event syntax error: 'open.c'
                     \___ Kernel verifier blocks program loading
<SNIP>
[root@jouet bpf]# 

The -v output looks the same:

[root@jouet bpf]# trace -v -e open,open.c touch /etc/passwd
bpf: builtin compilation failed: -95, try external compiler
Kernel build dir is set to /lib/modules/4.14.0-rc6+/build
set env: KBUILD_DIR=/lib/modules/4.14.0-rc6+/build
unset env: KBUILD_OPTS
include option is set to  -nostdinc -isystem /usr/lib/gcc/x86_64-redhat-linux/7/include -I/home/acme/git/linux/arch/x86/include -I./arch/x86/include/generated  -I/home/acme/git/linux/include -I./include -I/home/acme/git/linux/arch/x86/include/uapi -I./arch/x86/include/generated/uapi -I/home/acme/git/linux/include/uapi -I./include/generated/uapi -include /home/acme/git/linux/include/linux/kconfig.h 
set env: NR_CPUS=4
set env: LINUX_VERSION_CODE=0x40e00
set env: CLANG_EXEC=/usr/local/bin/clang
unset env: CLANG_OPTIONS
set env: KERNEL_INC_OPTIONS= -nostdinc -isystem /usr/lib/gcc/x86_64-redhat-linux/7/include -I/home/acme/git/linux/arch/x86/include -I./arch/x86/include/generated  -I/home/acme/git/linux/include -I./include -I/home/acme/git/linux/arch/x86/include/uapi -I./arch/x86/include/generated/uapi -I/home/acme/git/linux/include/uapi -I./include/generated/uapi -include /home/acme/git/linux/include/linux/kconfig.h 
set env: WORKING_DIR=/lib/modules/4.14.0-rc6+/build
set env: CLANG_SOURCE=/home/acme/bpf/open.c
llvm compiling command template: $CLANG_EXEC -D__KERNEL__ -D__NR_CPUS__=$NR_CPUS -DLINUX_VERSION_CODE=$LINUX_VERSION_CODE $CLANG_OPTIONS $KERNEL_INC_OPTIONS -Wno-unused-value -Wno-pointer-sign -working-directory $WORKING_DIR -c "$CLANG_SOURCE" -target bpf -O2 -o -
libbpf: loading object 'open.c' from buffer
libbpf: section .strtab, size 103, link 0, flags 0, type=3
libbpf: section .text, size 0, link 0, flags 6, type=1
libbpf: section prog=do_sys_open filename, size 184, link 0, flags 6, type=1
libbpf: found program prog=do_sys_open filename
libbpf: section .relprog=do_sys_open filename, size 16, link 8, flags 0, type=9
libbpf: section maps, size 16, link 0, flags 3, type=1
libbpf: section license, size 4, link 0, flags 3, type=1
libbpf: license of open.c is GPL
libbpf: section version, size 4, link 0, flags 3, type=1
libbpf: kernel version of open.c is 40e00
libbpf: section .symtab, size 144, link 1, flags 0, type=2
libbpf: maps in open.c: 1 maps in 16 bytes
libbpf: map 0 is "__bpf_stdout__"
libbpf: collecting relocating info for: 'prog=do_sys_open filename'
libbpf: relocation: insn_idx=15
libbpf: relocation: find map 0 (__bpf_stdout__) for insn 15
bpf: config program 'prog=do_sys_open filename'
symbol:do_sys_open file:(null) line:0 offset:0 return:0 lazy:(null)
parsing arg: filename into filename
bpf: config 'prog=do_sys_open filename' is ok
Looking at the vmlinux_path (8 entries long)
Using /lib/modules/4.14.0-rc6+/build/vmlinux for symbols
Open Debuginfo file: /lib/modules/4.14.0-rc6+/build/vmlinux
Try to find probe point from debuginfo.
Matched function: do_sys_open [2a2bbbe]
Probe point found: do_sys_open+0
Searching 'filename' variable in context.
Converting variable filename into trace event.
filename type is (null).
Opening /sys/kernel/debug/tracing//README write=0
Found 1 probe_trace_events.
Opening /sys/kernel/debug/tracing//kprobe_events write=1
Writing event: p:perf_bpf_probe/prog _text+2493152 filename=%si:x64
In map_prologue, ntevs=1
mapping[0]=0
libbpf: create map __bpf_stdout__: fd=3
prologue: pass validation
prologue: fast path
libbpf: load bpf program failed: Permission denied
libbpf: -- BEGIN DUMP LOG ---
libbpf: 
0: (79) r3 = *(u64 *)(r1 +104)
1: (b7) r2 = 0
2: (bf) r6 = r1
3: (bf) r1 = r10
4: (07) r1 += -128
5: (b7) r2 = 128
6: (85) call bpf_probe_read_str#45
7: (bf) r1 = r0
8: (07) r1 += -1
9: (67) r1 <<= 32
10: (77) r1 >>= 32
11: (25) if r1 > 0x7f goto pc+11
 R0=inv(id=0) R1=inv(id=0,umax_value=127,var_off=(0x0; 0x7f)) R6=ctx(id=0,off=0,imm=0) R10=fp0
12: (67) r0 <<= 32
13: (77) r0 >>= 32
14: (bf) r4 = r10
15: (07) r4 += -128
16: (bf) r1 = r6
17: (18) r2 = 0xffffa0b74ba91000
19: (18) r3 = 0xffffffff
21: (bf) r5 = r0
22: (85) call bpf_perf_event_output#25
invalid stack type R4 off=-128 access_size=0

libbpf: -- END LOG --
libbpf: Loading the 0th instance of program 'prog=do_sys_open filename' failed
libbpf: failed to load program 'prog=do_sys_open filename'
libbpf: failed to load object 'open.c'
bpf: load objects failed
event syntax error: 'open.c'
                     \___ Kernel verifier blocks program loading

Also:

[root@jouet bpf]# clang -v
clang version 4.0.0 (http://llvm.org/git/clang.git f5be8ba13adc4ba1011a7ccd60c844bd60427c1c) (http://llvm.org/git/llvm.git efca1a37676f4cd276d947658cf90b0fb625abfd)
Target: x86_64-unknown-linux-gnu
Thread model: posix
InstalledDir: /usr/local/bin
Found candidate GCC installation: /usr/lib/gcc/x86_64-redhat-linux/7
Selected GCC installation: /usr/lib/gcc/x86_64-redhat-linux/7
Candidate multilib: .;@m64
Candidate multilib: 32;@m32
Selected multilib: .;@m64
[root@jouet bpf]#

And now I've really attached that bpf.h header I use.

- Arnaldo