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Thu, 14 Nov 2024 16:59:50 +0000 Received: from BLAPR10MB5267.namprd10.prod.outlook.com ([fe80::682b:c879:9f97:a34f]) by BLAPR10MB5267.namprd10.prod.outlook.com ([fe80::682b:c879:9f97:a34f%3]) with mapi id 15.20.8158.017; Thu, 14 Nov 2024 16:59:44 +0000 Message-ID: Date: Thu, 14 Nov 2024 16:59:39 +0000 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v2 dwarves 1/2] dwarf_loader: Check DW_OP_[GNU_]entry_value for possible parameter matching To: Yonghong Song , acme@kernel.org Cc: dwarves@vger.kernel.org, ast@kernel.org, andrii@kernel.org, bpf@vger.kernel.org, daniel@iogearbox.net, kernel-team@fb.com, song@kernel.org, eddyz87@gmail.com, olsajiri@gmail.com References: <20241114155822.898466-1-alan.maguire@oracle.com> <20241114155822.898466-2-alan.maguire@oracle.com> <8a08219a-9312-429d-a291-d93a932c849a@linux.dev> Content-Language: en-GB From: Alan Maguire In-Reply-To: <8a08219a-9312-429d-a291-d93a932c849a@linux.dev> Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit X-ClientProxiedBy: AS4PR09CA0018.eurprd09.prod.outlook.com (2603:10a6:20b:5d4::6) To BLAPR10MB5267.namprd10.prod.outlook.com (2603:10b6:208:30e::22) Precedence: bulk X-Mailing-List: bpf@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 X-MS-PublicTrafficType: Email X-MS-TrafficTypeDiagnostic: BLAPR10MB5267:EE_|DS0PR10MB7342:EE_ X-MS-Office365-Filtering-Correlation-Id: 8c16e796-e6a1-41a0-f303-08dd04cdbd1e X-MS-Exchange-SenderADCheck: 1 X-MS-Exchange-AntiSpam-Relay: 0 X-Microsoft-Antispam: BCL:0;ARA:13230040|7416014|376014|10070799003|366016|1800799024; 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This happens >> in kernels 6.4, 6.9 and 6.11 and the kernel is built with pahole 1.27. >> >> The perf_event_read() signature in kernel (kernel/events/core.c): >>     static int perf_event_read(struct perf_event *event, bool group) >> >> Adding '-V' to pahole command line, and the following error msg can be >> found: >>     skipping addition of 'perf_event_read'(perf_event_read) due to >> unexpected register used for parameter >> >> Eventually the error message is attributed to the setting >> (parm->unexpected_reg = 1) in parameter__new() function. >> >> The following is the dwarf representation for perf_event_read(): >>      0x0334c034:   DW_TAG_subprogram >>                  DW_AT_low_pc    (0xffffffff812c6110) >>                  DW_AT_high_pc   (0xffffffff812c640a) >>                  DW_AT_frame_base        (DW_OP_reg7 RSP) >>                  DW_AT_GNU_all_call_sites        (true) >>                  DW_AT_name      ("perf_event_read") >>                  DW_AT_decl_file ("/rw/compile/kernel/events/core.c") >>                  DW_AT_decl_line (4641) >>                  DW_AT_prototyped        (true) >>                  DW_AT_type      (0x03324f6a "int") >>      0x0334c04e:     DW_TAG_formal_parameter >>                    DW_AT_location        (0x007de9fd: >>                       [0xffffffff812c6115, 0xffffffff812c6141): >> DW_OP_reg5 RDI >>                       [0xffffffff812c6141, 0xffffffff812c6323): >> DW_OP_reg14 R14 >>                       [0xffffffff812c6323, 0xffffffff812c63fe): >> DW_OP_GNU_entry_value(DW_OP_reg5 RDI), DW_OP_stack_value >>                       [0xffffffff812c63fe, 0xffffffff812c6405): >> DW_OP_reg14 R14 >>                       [0xffffffff812c6405, 0xffffffff812c640a): >> DW_OP_GNU_entry_value(DW_OP_reg5 RDI), DW_OP_stack_value) >>                    DW_AT_name    ("event") >>                    DW_AT_decl_file       ("/rw/compile/kernel/events/ >> core.c") >>                    DW_AT_decl_line       (4641) >>                    DW_AT_type    (0x0333aac2 "perf_event *") >>      0x0334c05e:     DW_TAG_formal_parameter >>                    DW_AT_location        (0x007dea82: >>                       [0xffffffff812c6137, 0xffffffff812c63f2): >> DW_OP_reg12 R12 >>                       [0xffffffff812c63f2, 0xffffffff812c63fe): >> DW_OP_GNU_entry_value(DW_OP_reg4 RSI), DW_OP_stack_value >>                       [0xffffffff812c63fe, 0xffffffff812c640a): >> DW_OP_reg12 R12) >>                    DW_AT_name    ("group") >>                    DW_AT_decl_file       ("/rw/compile/kernel/events/ >> core.c") >>                    DW_AT_decl_line       (4641) >>                    DW_AT_type    (0x03327059 "bool") >> >> By inspecting the binary, the second argument ("bool group") is used >> in the function. The following are the disasm code: >>      ffffffff812c6110 : >>      ffffffff812c6110: 0f 1f 44 00 00        nopl    (%rax,%rax) >>      ffffffff812c6115: 55                    pushq   %rbp >>      ffffffff812c6116: 41 57                 pushq   %r15 >>      ffffffff812c6118: 41 56                 pushq   %r14 >>      ffffffff812c611a: 41 55                 pushq   %r13 >>      ffffffff812c611c: 41 54                 pushq   %r12 >>      ffffffff812c611e: 53                    pushq   %rbx >>      ffffffff812c611f: 48 83 ec 18           subq    $24, %rsp >>      ffffffff812c6123: 41 89 f4              movl    %esi, %r12d >>      <=========== NOTE that here '%esi' is used and moved to '%r12d'. >>      ffffffff812c6126: 49 89 fe              movq    %rdi, %r14 >>      ffffffff812c6129: 65 48 8b 04 25 28 00 00 00    movq    %gs:40, %rax >>      ffffffff812c6132: 48 89 44 24 10        movq    %rax, 16(%rsp) >>      ffffffff812c6137: 8b af a8 00 00 00     movl    168(%rdi), %ebp >>      ffffffff812c613d: 85 ed                 testl   %ebp, %ebp >>      ffffffff812c613f: 75 3f                 jne     >> 0xffffffff812c6180 >>      ffffffff812c6141: 66 2e 0f 1f 84 00 00 00 00 00 nopw    %cs: >> (%rax,%rax) >>      ffffffff812c614b: 0f 1f 44 00 00        nopl    (%rax,%rax) >>      ffffffff812c6150: 49 8b 9e 28 02 00 00  movq    552(%r14), %rbx >>      ffffffff812c6157: 48 89 df              movq    %rbx, %rdi >>      ffffffff812c615a: e8 c1 a0 d7 00        callq   >> 0xffffffff82040220 <_raw_spin_lock_irqsave> >>      ffffffff812c615f: 49 89 c7              movq    %rax, %r15 >>      ffffffff812c6162: 41 8b ae a8 00 00 00  movl    168(%r14), %ebp >>      ffffffff812c6169: 85 ed                 testl   %ebp, %ebp >>      ffffffff812c616b: 0f 84 9a 00 00 00     je      >> 0xffffffff812c620b >>      ffffffff812c6171: 48 89 df              movq    %rbx, %rdi >>      ffffffff812c6174: 4c 89 fe              movq    %r15, %rsi >>      <=========== NOTE: %rsi is overwritten >>      ...... >>      ffffffff812c63f0: 41 5c                 popq    %r12 >>      <============ POP r12 >>      ffffffff812c63f2: 41 5d                 popq    %r13 >>      ffffffff812c63f4: 41 5e                 popq    %r14 >>      ffffffff812c63f6: 41 5f                 popq    %r15 >>      ffffffff812c63f8: 5d                    popq    %rbp >>      ffffffff812c63f9: e9 e2 a8 d7 00        jmp     >> 0xffffffff82040ce0 <__x86_return_thunk> >>      ffffffff812c63fe: 31 c0                 xorl    %eax, %eax >>      ffffffff812c6400: e9 be fe ff ff        jmp     >> 0xffffffff812c62c3 >> >> It is not clear why dwarf didn't encode %rsi in locations. But >> DW_OP_GNU_entry_value(DW_OP_reg4 RSI) tells us that RSI is live at >> the entry of perf_event_read(). So this patch tries to search >> DW_OP_GNU_entry_value/DW_OP_entry_value location/expression so if >> the expected parameter register matches the register in >> DW_OP_GNU_entry_value/DW_OP_entry_value, then the original parameter >> is not optimized. >> >> For one of internal 6.11 kernel, there are 62498 functions in BTF and >> perf_event_read() is not there. With this patch, there are 62552 >> functions >> in BTF and perf_event_read() is included. >> >> Reported-by: Song Liu >> Signed-off-by: Yonghong Song >> Signed-off-by: Eduard Zingerman >> Signed-off-by: Alan Maguire >> --- >>   dwarf_loader.c | 104 ++++++++++++++++++++++++++++++++++++++----------- >>   1 file changed, 81 insertions(+), 23 deletions(-) >> >> diff --git a/dwarf_loader.c b/dwarf_loader.c >> index ec8641b..bc862b5 100644 >> --- a/dwarf_loader.c >> +++ b/dwarf_loader.c >> @@ -1157,16 +1157,88 @@ static struct template_parameter_pack >> *template_parameter_pack__new(Dwarf_Die *d >>       return pack; >>   } >>   +/* Returns number of locations found or negative value for errors. */ >> +static ptrdiff_t __dwarf_getlocations(Dwarf_Attribute *attr, >> +                      ptrdiff_t offset, Dwarf_Addr *basep, >> +                      Dwarf_Addr *startp, Dwarf_Addr *endp, >> +                      Dwarf_Op **expr, size_t *exprlen) >> +{ >> +    int ret; >> + >> +#if _ELFUTILS_PREREQ(0, 157) >> +    ret = dwarf_getlocations(attr, offset, basep, startp, endp, expr, >> exprlen); >> +#else >> +    if (offset == 0) { >> +        ret = dwarf_getlocation(attr, expr, exprlen); >> +        if (ret == 0) >> +            ret = 1; >> +    } >> +#endif >> +    return ret; >> +} >> + >> +/* For DW_AT_location 'attr': >> + * - if first location is DW_OP_regXX with expected number, returns >> the register; >> + * - if location DW_OP_entry_value(DW_OP_regXX) is in the list, >> returns the register; >> + * - if first location is DW_OP_regXX, returns the register; >> + * - otherwise returns -1. >> + */ >> +static int parameter__reg(Dwarf_Attribute *attr, int expected_reg) >> +{ >> +    Dwarf_Addr base, start, end; >> +    Dwarf_Op *expr, *entry_ops; >> +    Dwarf_Attribute entry_attr; >> +    size_t exprlen, entry_len; >> +    ptrdiff_t offset = 0; >> +    int loc_num = -1; >> +    int ret = -1; >> + >> +    while ((offset = __dwarf_getlocations(attr, offset, &base, >> &start, &end, &expr, &exprlen)) > 0) { >> +        loc_num++; >> + >> +        /* Convert expression list (XX DW_OP_stack_value) -> (XX). >> +         * DW_OP_stack_value instructs interpreter to pop current >> value from >> +         * DWARF expression evaluation stack, and thus is not >> important here. >> +         */ >> +        if (exprlen > 1 && expr[exprlen - 1].atom == DW_OP_stack_value) >> +            exprlen--; >> + >> +        if (exprlen != 1) >> +            continue; >> + >> +        switch (expr->atom) { >> +        /* match DW_OP_regXX at first location */ >> +        case DW_OP_reg0 ... DW_OP_reg31: >> +            if (loc_num != 0) >> +                break; >> +            ret = expr->atom; >> +            if (expr->atom == expected_reg) >> +                goto out; >> +            break; >> +        /* match DW_OP_entry_value(DW_OP_regXX) at any location */ >> +        case DW_OP_entry_value: >> +        case DW_OP_GNU_entry_value: >> +            if (dwarf_getlocation_attr(attr, expr, &entry_attr) == 0 && >> +                dwarf_getlocation(&entry_attr, &entry_ops, >> &entry_len) == 0 && >> +                entry_len == 1) { >> +                ret = entry_ops->atom; > > Could we have more than one DW_OP_entry_value? What if the second one > matches execpted_reg? From dwarf5 documentation, there is no say about > whether we could have more than one DW_OP_entry_value or not. > > If we have evidence that only one DW_OP_entry_value will appear in > parameter > locations, a comment will be needed in the above. > > Otherwise, let us not do 'goto out' here. Rather, let us compare > entry_ops->atom with expected_reg. Do 'ret = entry_ops->atom' and > 'goto out' only if entry_ops->atom == expected_reg. Otherwise, > the original 'ret' value is preserved. > Makes sense. I'll wait for other feedback and roll that change into v3. Thanks! Alan