Add a simple script which disassembles a single function from an object
file.  Comes in handy for objtool warnings and kernel stack traces.

Originally-by: Peter Zijlstra <peterz@infradead.org>
Signed-off-by: Josh Poimboeuf <jpoimboe@redhat.com>
---
v2:
- require gawk

 scripts/objdump-func | 18 ++++++++++++++++++
 1 file changed, 18 insertions(+)
 create mode 100755 scripts/objdump-func

diff --git a/scripts/objdump-func b/scripts/objdump-func
new file mode 100755
index 000000000000..67229acfe3a6
--- /dev/null
+++ b/scripts/objdump-func
@@ -0,0 +1,18 @@
+#!/bin/bash
+# SPDX-License-Identifier: GPL-2.0
+#
+# Disassemble a single function.
+#
+# usage: objdump-func <file> <func>
+
+set -o errexit
+set -o nounset
+
+OBJDUMP="${CROSS_COMPILE:-}objdump"
+
+command -v gawk >/dev/null 2>&1 || die "gawk isn't installed"
+
+OBJ=$1; shift
+FUNC=$1; shift
+
+${OBJDUMP} -wdr $@ $OBJ | gawk "/^\$/ { P=0; } /$FUNC[^>]*>:\$/ { P=1; O=strtonum(\"0x\" \$1); } { if (P) { o=strtonum(\"0x\" \$1); printf(\"%04x \", o-O); print \$0; } }"
-- 
2.34.1

On Tue, Apr 19, 2022 at 09:12:23AM -0700, Josh Poimboeuf wrote:
> Add a simple script which disassembles a single function from an object
> file.  Comes in handy for objtool warnings and kernel stack traces.
> 
> Originally-by: Peter Zijlstra <peterz@infradead.org>
> Signed-off-by: Josh Poimboeuf <jpoimboe@redhat.com>
> ---
> v2:
> - require gawk
> 
>  scripts/objdump-func | 18 ++++++++++++++++++
>  1 file changed, 18 insertions(+)
>  create mode 100755 scripts/objdump-func

Was that supposed to be user-friendly too?

$ ./scripts/objdump-func -h
./scripts/objdump-func: line 16: $1: unbound variable

:-)


It works tho:

$ ./scripts/objdump-func vmlinux rewind_stack_and_make_dead
0000 ffffffff81001c00 <rewind_stack_and_make_dead>:
0000 ffffffff81001c00:  31 ed                   xor    %ebp,%ebp
0000 ffffffff81001c02:  65 48 8b 04 25 d0 ec 01 00      mov    %gs:0x1ecd0,%rax
0000 ffffffff81001c0b:  48 8d a0 58 ff ff ff    lea    -0xa8(%rax),%rsp
0000 ffffffff81001c12:  e8 39 24 07 00          call   ffffffff81074050 <make_task_dead>
^^^^

I guess those zeros at the beginning are supposed to mean something...

Thx.

-- 
Regards/Gruss,
    Boris.

https://people.kernel.org/tglx/notes-about-netiquette

On Tue, Apr 19, 2022 at 09:50:01PM +0200, Borislav Petkov wrote:
> On Tue, Apr 19, 2022 at 09:12:23AM -0700, Josh Poimboeuf wrote:
> > Add a simple script which disassembles a single function from an object
> > file.  Comes in handy for objtool warnings and kernel stack traces.
> > 
> > Originally-by: Peter Zijlstra <peterz@infradead.org>
> > Signed-off-by: Josh Poimboeuf <jpoimboe@redhat.com>
> > ---
> > v2:
> > - require gawk
> > 
> >  scripts/objdump-func | 18 ++++++++++++++++++
> >  1 file changed, 18 insertions(+)
> >  create mode 100755 scripts/objdump-func
> 
> Was that supposed to be user-friendly too?
> 
> $ ./scripts/objdump-func -h
> ./scripts/objdump-func: line 16: $1: unbound variable
> 
> :-)
> 
> 
> It works tho:
> 
> $ ./scripts/objdump-func vmlinux rewind_stack_and_make_dead
> 0000 ffffffff81001c00 <rewind_stack_and_make_dead>:
> 0000 ffffffff81001c00:  31 ed                   xor    %ebp,%ebp
> 0000 ffffffff81001c02:  65 48 8b 04 25 d0 ec 01 00      mov    %gs:0x1ecd0,%rax
> 0000 ffffffff81001c0b:  48 8d a0 58 ff ff ff    lea    -0xa8(%rax),%rsp
> 0000 ffffffff81001c12:  e8 39 24 07 00          call   ffffffff81074050 <make_task_dead>
> ^^^^
> 
> I guess those zeros at the beginning are supposed to mean something...

Use the .o file and it'll work:

$ ./objdump-func.sh defconfig-build/vmlinux.o rewind_stack_and_make_dead
0000 0000000000001380 <rewind_stack_and_make_dead>:
0000     1380:  31 ed                   xor    %ebp,%ebp
0002     1382:  65 48 8b 04 25 00 00 00 00      mov    %gs:0x0,%rax     1387: R_X86_64_32S      cpu_current_top_of_stack
000b     138b:  48 8d a0 58 ff ff ff    lea    -0xa8(%rax),%rsp
0012     1392:  e8 00 00 00 00          call   1397 <preempt_schedule_thunk>    1393: R_X86_64_PLT32    make_task_dead-0x4


Now the first coloumn is the symbol relative offset, while the second
column is the section relative offset objdump normally gives you.

On Tue, Apr 19, 2022 at 10:14:35PM +0200, Peter Zijlstra wrote:
> On Tue, Apr 19, 2022 at 09:50:01PM +0200, Borislav Petkov wrote:
> > On Tue, Apr 19, 2022 at 09:12:23AM -0700, Josh Poimboeuf wrote:
> > > Add a simple script which disassembles a single function from an object
> > > file.  Comes in handy for objtool warnings and kernel stack traces.
> > > 
> > > Originally-by: Peter Zijlstra <peterz@infradead.org>
> > > Signed-off-by: Josh Poimboeuf <jpoimboe@redhat.com>
> > > ---
> > > v2:
> > > - require gawk
> > > 
> > >  scripts/objdump-func | 18 ++++++++++++++++++
> > >  1 file changed, 18 insertions(+)
> > >  create mode 100755 scripts/objdump-func
> > 
> > Was that supposed to be user-friendly too?
> > 
> > $ ./scripts/objdump-func -h
> > ./scripts/objdump-func: line 16: $1: unbound variable
> > 
> > :-)

You mean you don't want to read the script to reverse engineer the usage
before running it???

> > It works tho:
> > 
> > $ ./scripts/objdump-func vmlinux rewind_stack_and_make_dead
> > 0000 ffffffff81001c00 <rewind_stack_and_make_dead>:
> > 0000 ffffffff81001c00:  31 ed                   xor    %ebp,%ebp
> > 0000 ffffffff81001c02:  65 48 8b 04 25 d0 ec 01 00      mov    %gs:0x1ecd0,%rax
> > 0000 ffffffff81001c0b:  48 8d a0 58 ff ff ff    lea    -0xa8(%rax),%rsp
> > 0000 ffffffff81001c12:  e8 39 24 07 00          call   ffffffff81074050 <make_task_dead>
> > ^^^^
> > 
> > I guess those zeros at the beginning are supposed to mean something...
> 
> Use the .o file and it'll work:

Should probably get fixed to work on vmlinux too...

-- 
Josh