[Crash-utility] [PATCH] crash/symbols: Error out earlier in case _stext_vmlinux is NULL or UNINITIALIZED

[Dave Anderson]

Hi Bhupesh,

I'm confused.  If the vmlinux cannot be found, then the crash session should
bail out after printing the standard error message in find_booted_kernel():

        error(INFO,
             "cannot find booted kernel -- please enter namelist argument\n\n");

If derive_kaslr_offset() is called, then it must have gotten past find_booted_kernel()
with a vmlinux file.  What am I missing?

Dave


----- Original Message -----
> While running crash on KASLR enabled live arm64 kernels I noticed
> that the crash would fail after printing the following error messages
> if it cannot find vmlinux for the boot'ed system:
> 
> crash: invalid kernel virtual address: 8470  type: "possible"
> WARNING: cannot read cpu_possible_map
> crash: invalid kernel virtual address: 8270  type: "present"
> WARNING: cannot read cpu_present_map
> crash: invalid kernel virtual address: 8070  type: "online"
> WARNING: cannot read cpu_online_map
> crash: invalid kernel virtual address: 8670  type: "active"
> WARNING: cannot read cpu_active_map
> 
> crash: cannot resolve "_stext"
> 
> Since vmlinux is not available at the standard expected location, i.e
> '/usr/lib/debug/lib/modules/<kernel-version>' directory, so
> _stext_vmlinux stays UNINITIALIZED inside 'derive_kaslr_offset()'
> and 'relocate' becomes 0.
> 
> Later-on this can lead to 'cannot resolve "_stext"' error in
> 'symbol_value()' function.
> 
> We can go ahead and fix the same, but ideally since we are running
> on a live system, we should be finding the vmlinux at the standard
> location, otherwise we should do proper error handling.
> 
> So this patch errors out earlier, if we don't find a valid value for
> '_stext_vmlinux' and prompts the user to refer to the 'usage'
> banner.
> 
> Signed-off-by: Bhupesh Sharma <bhsharma at redhat.com>
> ---
>  symbols.c | 18 ++++++++++++++++++
>  1 file changed, 18 insertions(+)
> 
> diff --git a/symbols.c b/symbols.c
> index 4db9af7bf044..30669950635d 100644
> --- a/symbols.c
> +++ b/symbols.c
> @@ -663,6 +663,24 @@ derive_kaslr_offset(bfd *abfd, int dynamic, bfd_byte
> *start, bfd_byte *end,
>  			return;
>  	}
>  
> +	/* For KASLR enabled live kernels, if vmlinux is not available
> +	 * at the standard expected location, i.e
> +	 * '/usr/lib/debug/lib/modules/<kernel-version>' directory,
> +	 * then _stext_vmlinux will be set to NULL or UNINITIALIZED.
> +	 *
> +	 * Later-on this may lead to "_stext" symbol not being resolved
> +	 * properly (which may lead to an ambiguous error message).
> +	 *
> +	 * So, error out earlier here itself, in case _stext_vmlinux
> +	 * is NULL or UNINITIALIZED.
> +	 */
> +	if (!st->_stext_vmlinux || (st->_stext_vmlinux == UNINITIALIZED)) {
> +		error(INFO, "_stext not found in vmlinux :\n"
> +		      "   if running a live system -- please provide a suitable vmlinux,\n"
> +		      "   otherwise -- please enter namelist argument.\n");
> +		program_usage(SHORT_FORM);
> +	}
> +
>  	/*
>  	 * To avoid mistaking an mismatched kernel version with
>  	 * a kaslr offset, we make sure that the offset is
> --
> 2.7.4
> 
>