Re: Issue using faddr2line on kernel modules

[Josh Poimboeuf <jpoimboe@redhat.com>]

On Mon, Jan 17, 2022 at 11:48:39AM -0800, Josh Poimboeuf wrote:
> On Mon, Jan 17, 2022 at 10:27:14AM +0530, Kaiwan N Billimoria wrote:
> > Hi there,
> > 
> > Am researching using the cool faddr2line script to help debug Oops'es
> > from kernel modules..
> > I find it works just fine when used against the unstripped vmlinux
> > with debug symbols.
> > 
> > My use case is for a kernel module which Oopses, though. Here's my scenario:
> > I built a module on a custom debug kernel (5.10.60) with most debug
> > options enabled...
> > KASLR is enabled by default as well.
> > 
> > A test kernel module Oopses on my x86_64 guest running this kernel with:
> > RIP: 0010:do_the_work+0x15b/0x174 [oops_tryv2]
> > 
> > So, i try this:
> > 
> > $ <...>/linux-5.10.60/scripts/faddr2line ./oops_tryv2.ko do_the_work+0x15b/0x174
> > bad symbol size: base: 0x0000000000000000 end: 0x0000000000000000
> > $
> > 
> > (It works fine with addr2line though!).
> > Now I think I've traced the faddr2line script's failure to locate
> > anything down to this:
> > ...
> > done < <(${NM} -n $objfile | awk -v fn=$func -v end=$file_end '$3 ==
> > fn { found=1; line    =$0; start=$1; next } found == 1 { found=0;
> > print line, "0x"$1 } END {if (found == 1) print     line, end; }')
> > 
> > The nm output is:
> > $ nm -n ./oops_tryv2.ko |grep -i do_the_work
> > 0000000000000000 t do_the_work
> > $
> > 
> > nm shows the text addr as 0x0; this is obviously incorrect (same 0x0
> > with objdump -d on the module).
> > Am I missing something? Any suggestions as to what I can try, to get
> > faddr2line working?
> 
> Hi Kaiwan,
> 
> Thanks for reporting this issue.  The module text address of 0x0 is not
> necessarily incorrect, as the address is relative the the module, where
> all text usually starts at zero.
> 
> I was able to recreate this problem using a module which only has a
> single function in .text.  Does this fix it?

Actually, that patch has other problems.  Try this one?

----

From: Josh Poimboeuf <jpoimboe@redhat.com>
Subject: [PATCH] scripts/faddr2line: Only look for text symbols when
 calculating function size

With the following commit:

  efdb4167e676 ("scripts/faddr2line: Fix "size mismatch" error")

... it was discovered that faddr2line can't just read a function's ELF
size, because that wouldn't match the kallsyms function size which is
printed in the stack trace.  The kallsyms size includes any padding
after the function, whereas the ELF size does not.

So faddr2line has to manually calculate the size of a function similar
to how kallsyms does.  It does so by starting with a sorted list of
symbols and subtracting the function address from the subsequent
symbol's address.

That calculation is broken in the case where the function is the last
(or only) symbol in the .text section.  The next symbol in the sorted
list might actually be a data symbol, which can break the function size
detection:

  $ scripts/faddr2line sound/soundcore.ko sound_devnode+0x5/0x35
  bad symbol size: base: 0x0000000000000000 end: 0x0000000000000000

Similar breakage can occur when reading from a .o file.

Fix it by only looking for text symbols.

Fixes: efdb4167e676 ("scripts/faddr2line: Fix "size mismatch" error")
Reported-by: Kaiwan N Billimoria <kaiwan.billimoria@gmail.com>
Signed-off-by: Josh Poimboeuf <jpoimboe@redhat.com>
---
 scripts/faddr2line | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/scripts/faddr2line b/scripts/faddr2line
index 6c6439f69a72..2a130134f1e6 100755
--- a/scripts/faddr2line
+++ b/scripts/faddr2line
@@ -189,7 +189,7 @@ __faddr2line() {
 
 		DONE=1
 
-	done < <(${NM} -n $objfile | awk -v fn=$func -v end=$file_end '$3 == fn { found=1; line=$0; start=$1; next } found == 1 { found=0; print line, "0x"$1 } END {if (found == 1) print line, end; }')
+	done < <(${NM} -n $objfile | awk -v fn=$func -v end=$file_end '$2 !~ /[Tt]/ {next} $3 == fn { found=1; line=$0; start=$1; next } found == 1 { found=0; print line, "0x"$1 } END {if (found == 1) print line, end; }')
 }
 
 [[ $# -lt 2 ]] && usage
-- 
2.31.1