Re: Issue using faddr2line on kernel modules

[Josh Poimboeuf <jpoimboe@redhat.com>]

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?

diff --git a/scripts/faddr2line b/scripts/faddr2line
index 6c6439f69a72..cd6e812b9d06 100755
--- a/scripts/faddr2line
+++ b/scripts/faddr2line
@@ -111,21 +111,11 @@ __faddr2line() {
 
 	# Go through each of the object's symbols which match the func name.
 	# In rare cases there might be duplicates.
-	file_end=$(${SIZE} -Ax $objfile | awk '$1 == ".text" {print $2}')
 	while read symbol; do
 		local fields=($symbol)
 		local sym_base=0x${fields[0]}
-		local sym_type=${fields[1]}
-		local sym_end=${fields[3]}
-
-		# calculate the size
-		local sym_size=$(($sym_end - $sym_base))
-		if [[ -z $sym_size ]] || [[ $sym_size -le 0 ]]; then
-			warn "bad symbol size: base: $sym_base end: $sym_end"
-			DONE=1
-			return
-		fi
-		sym_size=0x$(printf %x $sym_size)
+		local sym_size=0x${fields[1]}
+		local sym_type=${fields[2]}
 
 		# calculate the address
 		local addr=$(($sym_base + $offset))
@@ -189,7 +179,7 @@ __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} -nS $objfile | awk -v fn=$func '$4 == fn')
 }
 
 [[ $# -lt 2 ]] && usage