Re: vmlinux buildid doesn't match the running one (/sys/kernel/notes)

[Lin Ming <ming.m.lin@intel.com>]

> From: Francis Moreau <francis.moro@gmail.com>
> Date: Mon, May 2, 2011 at 3:02 AM
> Subject: Re: vmlinux buildid doesn't match the running one (/sys/kernel/notes)
> To: linux-perf-users@vger.kernel.org
> 
> 
> Hi,
> 
> Sorry for asking again but I would be really greatfull is someone can
> tell me what's wrong when using a kernel that I compiled by my own.
> 
> I do not have that issue is I use the kernel shipped by my distro (FC14).
> 
> Thanks
> 
> On Thu, Apr 28, 2011 at 7:04 PM, Francis Moreau <francis.moro@gmail.com> wrote:
> > Hello,
> >
> > I'm trying to use perf-probe, but it fails.
> >
> > Basically it's due to the buildid of the running kernel (the one which
> > can be read from /sys/kernel/notes) and the one in vmlinx (which I
> > compiled my self).
> >
> > I did this:
> >
> >  $ make
> >  $ make modules_install install
> >  $ ls vmlinux
> >  vmlinux
> >
> > reboot

Could you make sure you indeed reboot with the new kernel running?

Use below patch to dump the 2 build_ids.

diff --git a/tools/perf/util/symbol.c b/tools/perf/util/symbol.c
index 516876d..4b66c1c 100644
--- a/tools/perf/util/symbol.c
+++ b/tools/perf/util/symbol.c
@@ -1036,6 +1036,15 @@ static size_t elf_addr_to_index(Elf *elf, GElf_Addr addr)
 	return -1;
 }
 
+static void dump_build_id(u8 *build_id)
+{
+	int i;
+
+	for (i = 0; i < BUILD_ID_SIZE; i++)
+		printf("%02x ", build_id[i]);
+	printf("\n");
+}
+
 static int dso__load_sym(struct dso *dso, struct map *map, const char *name,
 			 int fd, symbol_filter_t filter, int kmodule,
 			 int want_symtab)
@@ -1075,8 +1084,11 @@ static int dso__load_sym(struct dso *dso, struct map *map, const char *name,
 				      BUILD_ID_SIZE) != BUILD_ID_SIZE)
 			goto out_elf_end;
 
-		if (!dso__build_id_equal(dso, build_id))
+		if (!dso__build_id_equal(dso, build_id)) {
+			dump_build_id(build_id);
+			dump_build_id(dso->build_id);
 			goto out_elf_end;
+		}
 	}
 
 	sec = elf_section_by_name(elf, &ehdr, &shdr, ".symtab", NULL);


> >
> >  $ perf probe -k vmlinux -L schedule
> >  Failed to find path of kernel module.
> >  Failed to open debuginfo file.
> >  Error: Failed to show lines. (-2)
> >
> > So my question is why the buildids doesn't match ?
> >
> > I guess I did something wrong since it's pretty a standard use case
> > but I don't see why...
> >
> > Thanks
> > --
> > Francis