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

From: Namhyung Kim
Date: Mon Feb 05 2024 - 21:22:15 EST

On Sun, Feb 4, 2024 at 10:58 PM Adrian Hunter <adrian.hunter@xxxxxxxxx> wrote:
>
> 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-elfc
> >> 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*.

Ok, thanks for the explanation!

Namhyung