[Crash-utility] Can't open i386 vmcore with crash on 2.6.20-rc2kernels

[Vivek Goyal]

On Wed, Jan 03, 2007 at 09:29:52AM -0500, Dave Anderson wrote:
> >
> > WARNING: invalid linux_banner pointer: 756e694c
> > crash: vmlinux and vmcore do not match!
> >
> 
> Hi Vivek,
> 
> It has to do with how the linux_banner symbol is declared.  The crash code
> in question does this:
> 
> 
>         if (!(sp = symbol_search("linux_banner")))
>                 error(FATAL, "linux_banner symbol does not exist?\n");
>         else if (sp->type == 'R')
>                 linux_banner = symbol_value("linux_banner");
>         else
>                 get_symbol_data("linux_banner", sizeof(ulong), &linux_banner);
> 
>         if (!IS_KVADDR(linux_banner))
>                 error(WARNING, "invalid linux_banner pointer: %lx\n",
>                         linux_banner);
> 
> Up until 2.6.20-rc2 (apparently), if the linux_banner symbol
> was in the readonly section ('R'), the string data was located
> at the address of the linux_banner symbol.  Otherwise, the
> linux_banner symbol contained the address of the string data.
> 
> In your kernel, what does "nm -Bn vmlinux | grep linux_banner" show?
> Maybe 'r' instead of 'R'?
> 

Hi Dave,

You are right. Following is "nm -Bn vmlinux | grep linux_banner" output.

c0411000 r linux_banner

I checked that in 2.6.19, following was definition of linux_banner.

const char linux_banner[] =
        "Linux version " UTS_RELEASE " (" LINUX_COMPILE_BY "@"
        LINUX_COMPILE_HOST ") (" LINUX_COMPILER ") " UTS_VERSION "\n";

Symbol table entry for linux_banner:
c0407000   129 OBJECT  GLOBAL DEFAULT    3 linux_banner

But in 2.6.20-rc, it has been made static. Hence scope of this symbol is no
more global and now it is local, hence 'r' instead of 'R'.

static const char linux_banner[] =
        "Linux version " UTS_RELEASE
        " (" LINUX_COMPILE_BY "@" LINUX_COMPILE_HOST ")"
        " (" LINUX_COMPILER ")"
        " " UTS_VERSION "\n";

Symbol table entry for linux_banner:

c0411000   143 OBJECT  LOCAL  DEFAULT    7 linux_banner

Attached test patch works for me.

Just a thought, can debug info section tell us what is the type of
linux_banner? I mean something to differentiate between above two cases
where linux_banner itself is a string or it contains a pointer to string.

Thanks
Vivek



Signed-off-by: Vivek Goyal <vgoyal at in.ibm.com>
---

 kernel.c |    2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff -puN kernel.c~crash-unable-to-read-linux-banner-fix kernel.c
--- crash-4.0-3.16/kernel.c~crash-unable-to-read-linux-banner-fix	2007-01-04 14:05:23.000000000 +0530
+++ crash-4.0-3.16-root/kernel.c	2007-01-04 14:07:15.000000000 +0530
@@ -480,7 +480,7 @@ verify_version(void)
 
 	if (!(sp = symbol_search("linux_banner")))
 		error(FATAL, "linux_banner symbol does not exist?\n");
-	else if (sp->type == 'R')
+	else if ((sp->type == 'R') || (sp->type == 'r'))
 		linux_banner = symbol_value("linux_banner");
 	else
 		get_symbol_data("linux_banner", sizeof(ulong), &linux_banner);
_