Re: [PATCH 2/2] perf symbols: Slightly improve module file executable section mappings

[Adrian Hunter]

On 3/02/24 03:44, Namhyung Kim wrote:
> Hi Adrian,
> 
> On Fri, Feb 02, 2024 at 01:01:30PM +0200, Adrian Hunter wrote:
>> Currently perf does not record module section addresses except for
>> the .text section. In general that means perf cannot get module section
>> mappings correct (except for .text) when loading symbols from a kernel
>> module file. (Note using --kcore does not have this issue)
>>
>> Improve that situation slightly by identifying executable sections that
>> use the same mapping as the .text section. That happens when an
>> executable section comes directly after the .text section, both in memory
>> and on file, something that can be determined by following the same layout
>> rules used by the kernel, refer kernel layout_sections(). Note whether
>> that happens is somewhat arbitrary, so this is not a final solution.
>>
>> Example from tracing a virtual machine process:
>>
>>  Before:
>>
>>   $ perf script | grep unknown
>>          CPU 0/KVM    1718   203.511270:     318341 cpu-cycles:P:  ffffffffc13e8a70 [unknown] (/lib/modules/6.7.2-local/kernel/arch/x86/kvm/kvm-intel.ko)
>>   $ perf script -vvv 2>&1 >/dev/null | grep kvm.intel | grep 'noinstr.text\|ffff'
>>   Map: 0-7e0 41430 [kvm_intel].noinstr.text
>>   Map: ffffffffc13a7000-ffffffffc1421000 a0 /lib/modules/6.7.2-local/kernel/arch/x86/kvm/kvm-intel.ko
>>
>>  After:
>>
>>   $ perf script | grep 203.511270
>>          CPU 0/KVM    1718   203.511270:     318341 cpu-cycles:P:  ffffffffc13e8a70 vmx_vmexit+0x0 (/lib/modules/6.7.2-local/kernel/arch/x86/kvm/kvm-intel.ko)
>>   $ perf script -vvv 2>&1 >/dev/null | grep kvm.intel | grep 'noinstr.text\|ffff'
>>   Map: ffffffffc13a7000-ffffffffc1421000 a0 /lib/modules/6.7.2-local/kernel/arch/x86/kvm/kvm-intel.ko
>>
>> Reported-by: Like Xu <like.xu.linux@xxxxxxxxx>
>> Signed-off-by: Adrian Hunter <adrian.hunter@xxxxxxxxx>
>> ---
>>  tools/perf/util/symbol-elf.c | 75 +++++++++++++++++++++++++++++++++++-
>>  1 file changed, 73 insertions(+), 2 deletions(-)
>>
>> diff --git a/tools/perf/util/symbol-elf.c b/tools/perf/util/symbol-elf.c
>> index 9e7eeaf616b8..98bf0881aaf6 100644
>> --- a/tools/perf/util/symbol-elf.c
>> +++ b/tools/perf/util/symbol-elf.c
>> @@ -23,6 +23,7 @@
>>  #include <linux/ctype.h>
>>  #include <linux/kernel.h>
>>  #include <linux/zalloc.h>
>> +#include <linux/string.h>
>>  #include <symbol/kallsyms.h>
>>  #include <internal/lib.h>
>>  
>> @@ -1329,6 +1330,58 @@ int symsrc__init(struct symsrc *ss, struct dso *dso, const char *name,
>>  	return -1;
>>  }
>>  
>> +static bool is_exe_text(int flags)
>> +{
>> +	return (flags & (SHF_ALLOC | SHF_EXECINSTR)) == (SHF_ALLOC | SHF_EXECINSTR);
>> +}
>> +
>> +/*
>> + * Some executable module sections like .noinstr.text might be laid out with
>> + * .text so they can use the same mapping (memory address to file offset).
>> + * Check if that is the case. Refer to kernel layout_sections(). Return the
>> + * maximum offset.
>> + */
>> +static u64 max_text_section(Elf *elf, GElf_Ehdr *ehdr)
>> +{
>> +	Elf_Scn *sec = NULL;
>> +	GElf_Shdr shdr;
>> +	u64 offs = 0;
>> +
>> +	/* Doesn't work for some arch */
>> +	if (ehdr->e_machine == EM_PARISC ||
>> +	    ehdr->e_machine == EM_ALPHA)
>> +		return 0;
>> +
>> +	/* ELF is corrupted/truncated, avoid calling elf_strptr. */
>> +	if (!elf_rawdata(elf_getscn(elf, ehdr->e_shstrndx), NULL))
>> +		return 0;
>> +
>> +	while ((sec = elf_nextscn(elf, sec)) != NULL) {
>> +		char *sec_name;
>> +
>> +		if (!gelf_getshdr(sec, &shdr))
>> +			break;
>> +
>> +		if (!is_exe_text(shdr.sh_flags))
>> +			continue;
>> +
>> +		/* .init and .exit sections are not placed with .text */
>> +		sec_name = elf_strptr(elf, ehdr->e_shstrndx, shdr.sh_name);
>> +		if (!sec_name ||
>> +		    strstarts(sec_name, ".init") ||
>> +		    strstarts(sec_name, ".exit"))
>> +			break;
> 
> Do we really need this?  It seems my module has .init.text section
> next to .text.
> 
>   $ readelf -SW /lib/modules/`uname -r`/kernel/fs/ext4/ext4.ko
>   There are 77 section headers, starting at offset 0x252e90:
>   
>   Section Headers:
>     [Nr] Name              Type            Address          Off    Size   ES Flg Lk Inf Al
>     [ 0]                   NULL            0000000000000000 000000 000000 00      0   0  0
>     [ 1] .text             PROGBITS        0000000000000000 000040 079fa7 00  AX  0   0 16
>     [ 2] .rela.text        RELA            0000000000000000 13c348 04f0c8 18   I 74   1  8
>     [ 3] .init.text        PROGBITS        0000000000000000 079ff0 00060c 00  AX  0   0 16
>     ...
> 
> 
> ALIGN(0x40 + 0x79fa7, 16) = 0x79ff0, right?

But not in memory e.g.

Section Headers:
  [Nr] Name              Type            Address          Off    Size   ES Flg Lk Inf Al
  [ 3] .text             PROGBITS        0000000000000000 0000a0 071719 00  AX  0   0 16
  [ 5] .text.unlikely    PROGBITS        0000000000000000 0717b9 000a59 00  AX  0   0  1
  [ 7] .init.text        PROGBITS        0000000000000000 072212 0004fe 00  AX  0   0  1
  [ 9] .altinstr_replacement PROGBITS    0000000000000000 072710 000004 00  AX  0   0  1
  [10] .static_call.text PROGBITS        0000000000000000 072714 000388 00  AX  0   0  4
  [12] .exit.text        PROGBITS        0000000000000000 072a9c 000078 00  AX  0   0  1


/sys/module/ext4/sections/.text:                 0xffffffffc0453000
/sys/module/ext4/sections/.text.unlikely:        0xffffffffc04c4719
/sys/module/ext4/sections/.init.text:            0xffffffffc053e000
/sys/module/ext4/sections/.altinstr_replacement: 0xffffffffc04c5172
/sys/module/ext4/sections/.static_call.text:     0xffffffffc04c5178
/sys/module/ext4/sections/.exit.text:            0xffffffffc04c5500

Need to have:

	section address - offset == .text address - .text offset

perf does not record the section address, but the kernel
layout_sections() lays out executable sections in order
starting with .text *until* it gets to .init* or .exit*.