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SubjectRe: [PATCH] perf bpf: Fix endianness problem when loading parameters in prologue
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On 08/11/2017 09:23 PM, Arnaldo Carvalho de Melo wrote:
> Em Fri, Aug 11, 2017 at 06:47:56PM +0800, Wangnan (F) escreveu:
>> Hi Thomas,
>>
>> Please try this patch on your machine and give me the result.
>
> Right, I'm waiting for test results for the last two patches from Wang:
>
> (3.0K) [PATCH] perf bpf: Fix endianness problem when loading parameters in prologue

This patch does not solve the issue completely. It solves load of parameter 1 which is
loaded from 132(%r2):x32 from memory via function BPF_FUNC_probe_read.

How another issue is introduced with register r3:
The probe_events: p:perf_bpf_probe/func _text+8769704 f_mode=+132(%r2):x32
offset=%r3:s64 orig=%r4:s32
Now r4 does not have any offset at all and is not read from memory via
function BPF_FUNC_probe_read. The value is instead stored in a 8 byte register.

When I look into the generated eBPF bytes code I see

(1) r3 = *(r6 + 56) <-- R3 loaded from ctx pointer + 56
which is value of register r3 (origin of lseek)
....
(2) *(r10 - 24) = r3 <-- R3 is stored on stack position for function call)
...
(3) r3 = *(r10 - 8) <-- load f_mode into r3
(4) r4 = *(r10 - 16) <-- load offset into r4
(5) r5 = *(r10 - 24) <-- load origin into r5

I have provided a reworked patch to solve this issue. Please review it because
I do not know if it will work in all cases.
See below.


> (1.2K) [PATCH] perf test llvm: Fix f_mode endianness problem
>
> Thanks,
>
> - Arnaldo
>
>> Thank you.
>>
>> On 2017/8/13 2:49, Wang Nan wrote:
>>> Perf BPF prologue generator unconditionally fetches 8 bytes for function
>>> parameters, which causes problem on big endian machine. Thomas gives a
>>> detail analysis for this problem:
>>>
>>> http://lkml.kernel.org/r/968ebda5-abe4-8830-8d69-49f62529d151@linux.vnet.ibm.com
>>>
>>> This patch parses the type of each argument and converts data from
>>> memory to expected type.
>>>
>>> Signed-off-by: Wang Nan <wangnan0@huawei.com>
>>> Cc: Arnaldo Carvalho de Melo <acme@kernel.org>
>>> Cc: Thomas Richter <tmricht@linux.vnet.ibm.com>
>>> Cc: Alexei Starovoitov <alexei.starovoitov@gmail.com>
>>> Cc: Hendrik Brueckner <brueckner@linux.vnet.ibm.com>
>>> Cc: Li Zefan <lizefan@huawei.com>
>>> ---
>>> tools/perf/tests/bpf-script-test-prologue.c | 4 ++-
>>> tools/perf/util/bpf-prologue.c | 49 +++++++++++++++++++++++++++--
>>> 2 files changed, 50 insertions(+), 3 deletions(-)
>>>
>>> diff --git a/tools/perf/tests/bpf-script-test-prologue.c b/tools/perf/tests/bpf-script-test-prologue.c
>>> index b4ebc75..43f1e16 100644
>>> --- a/tools/perf/tests/bpf-script-test-prologue.c
>>> +++ b/tools/perf/tests/bpf-script-test-prologue.c
>>> @@ -26,9 +26,11 @@ static void (*bpf_trace_printk)(const char *fmt, int fmt_size, ...) =
>>> (void *) 6;
>>> SEC("func=null_lseek file->f_mode offset orig")
>>> -int bpf_func__null_lseek(void *ctx, int err, unsigned long f_mode,
>>> +int bpf_func__null_lseek(void *ctx, int err, unsigned long _f_mode,
>>> unsigned long offset, unsigned long orig)
>>> {
>>> + fmode_t f_mode = (fmode_t)_f_mode;
>>> +
>>> if (err)
>>> return 0;
>>> if (f_mode & FMODE_WRITE)
>>> diff --git a/tools/perf/util/bpf-prologue.c b/tools/perf/util/bpf-prologue.c
>>> index 6cdbee1..ce28993 100644
>>> --- a/tools/perf/util/bpf-prologue.c
>>> +++ b/tools/perf/util/bpf-prologue.c
>>> @@ -57,6 +57,46 @@ check_pos(struct bpf_insn_pos *pos)
>>> return 0;
>>> }
>>> +/*
>>> + * Convert type string (u8/u16/u32/u64/s8/s16/s32/s64 ..., see
>>> + * Documentation/trace/kprobetrace.txt) to size field of BPF_LDX_MEM
>>> + * instruction (BPF_{B,H,W,DW}).
>>> + */
>>> +static int
>>> +argtype_to_ldx_size(const char *type)
>>> +{
>>> + int arg_size = type ? atoi(&type[1]) : 64;
>>> +
>>> + switch (arg_size) {
>>> + case 8:
>>> + return BPF_B;
>>> + case 16:
>>> + return BPF_H;
>>> + case 32:
>>> + return BPF_W;
>>> + case 64:
>>> + default:
>>> + return BPF_DW;
>>> + }
>>> +}
>>> +
>>> +static const char *
>>> +insn_sz_to_str(int insn_sz)
>>> +{
>>> + switch (insn_sz) {
>>> + case BPF_B:
>>> + return "BPF_B";
>>> + case BPF_H:
>>> + return "BPF_H";
>>> + case BPF_W:
>>> + return "BPF_W";
>>> + case BPF_DW:
>>> + return "BPF_DW";
>>> + default:
>>> + return "UNKNOWN";
>>> + }
>>> +}
>>> +
>>> /* Give it a shorter name */
>>> #define ins(i, p) append_insn((i), (p))
>>> @@ -257,9 +297,14 @@ gen_prologue_slowpath(struct bpf_insn_pos *pos,
>>> }
>>> /* Final pass: read to registers */
>>> - for (i = 0; i < nargs; i++)
>>> - ins(BPF_LDX_MEM(BPF_DW, BPF_PROLOGUE_START_ARG_REG + i,
>>> + for (i = 0; i < nargs; i++) {
>>> + int insn_sz = argtype_to_ldx_size(args[i].type);

int insn_sz = (args[i].ref) ? argtype_to_ldx_size(args[i].type) : BPF_DW;

>>> +
>>> + pr_debug("prologue: load arg %d, insn_sz is %s\n",
>>> + i, insn_sz_to_str(insn_sz));
>>> + ins(BPF_LDX_MEM(insn_sz, BPF_PROLOGUE_START_ARG_REG + i,
>>> BPF_REG_FP, -BPF_REG_SIZE * (i + 1)), pos);
>>> + }
>>> ins(BPF_JMP_IMM(BPF_JA, BPF_REG_0, 0, JMP_TO_SUCCESS_CODE), pos);
>>
>


--
Thomas Richter, Dept 3303, IBM LTC Boeblingen Germany
--
Vorsitzende des Aufsichtsrats: Martina Koederitz
Geschäftsführung: Dirk Wittkopp
Sitz der Gesellschaft: Böblingen / Registergericht: Amtsgericht Stuttgart, HRB 243294

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