Messages in this thread | | | Date | Mon, 5 Feb 2024 08:58:05 +0200 | Subject | Re: [PATCH 2/2] perf symbols: Slightly improve module file executable section mappings | From | Adrian Hunter <> |
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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@gmail.com> >> Signed-off-by: Adrian Hunter <adrian.hunter@intel.com> >> --- >> 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*.
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