linux-kernel - Re: [PATCH] Fix implicit cast warning in test_klp

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Date: Wed, 21 Feb 2024 13:26:57 -0500
From: Joe Lawrence <joe.lawrence@...hat.com>
To: Nicolai Stange <nstange@...e.de>,
 Shresth Prasad <shresthprasad7@...il.com>
Cc: zhangwarden@...il.com, jikos@...nel.org, jpoimboe@...nel.org,
 linux-kernel-mentees@...ts.linuxfoundation.org,
 linux-kernel@...r.kernel.org, linux-kselftest@...r.kernel.org,
 live-patching@...r.kernel.org, mbenes@...e.cz, mpdesouza@...e.com,
 pmladek@...e.com, shuah@...nel.org, skhan@...uxfoundation.org
Subject: Re: [PATCH] Fix implicit cast warning in test_klp_state.c

On 2/21/24 07:44, Nicolai Stange wrote:
> Shresth Prasad <shresthprasad7@...il.com> writes:
> 
>> I checked the source code and yes I am on the latest Linux next repo.
>>
>> Here's the warning:
>> /home/shresthp/dev/linux_work/linux_next/tools/testing/selftests/livepatch/test_modules/test_klp_state.c:38:24: warning: assignment to ‘struct klp_state *’ from ‘int’ makes pointer from integer without a cast [-Wint-conversion]
>>    38 |         loglevel_state = klp_get_state(&patch, CONSOLE_LOGLEVEL_STATE);
>>       |                        ^
> 
> 
> Is the declaration of klp_get_state() visible at that point, i.e. is
> there perhaps any warning about missing declarations above that?
> 
> Otherwise C rules would default to assume an 'int' return type.
> 

This is an interesting clue.  I thought I might be able to reproduce the
build error by modifying include/livepatch.h and running `make -j15 -C
tools/testing/selftests/livepatch` ... but that seemed to work fine on
my system.  I even removed the entire include/ subdir from my tree and
it still built the test module.  Huh?

Then I moved /lib/modules/$(uname -r)/build out of the way and saw that
the compilation failed.  Ah hah -- that's right, it's using the system
build tree.  That version of livepatch.h may have a missing or
completely different definition of klp_get_state().

How does this sequence work for you, Shresth:

  # Verify that kernel livepatching is turned on
  $ grep LIVEPATCH .config
  CONFIG_HAVE_LIVEPATCH=y
  CONFIG_LIVEPATCH=y

  # Build linux-next kernel tree and then the livepatch selftests,
  # pointing KDIR to this tree
  $ make -j$(nproc) vmlinux && \
    make -j$(nproc) KDIR=$(pwd) -C tools/testing/selftests/livepatch

-- 
Joe