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From: Daniel Axtens <dja@axtens.net>
To: Lakshmi Ramasubramanian <nramas@linux.microsoft.com>, robh@kernel.org,
 dan.carpenter@oracle.com
Subject: Re: [PATCH] powerpc: Initialize local variable fdt to NULL in
 elf64_load()
In-Reply-To: <87eefag241.fsf@linkitivity.dja.id.au>
References: <20210415191437.20212-1-nramas@linux.microsoft.com>
 <4edb1433-4d1e-5719-ec9c-fd232b7cf71f@linux.microsoft.com>
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Date: Thu, 22 Apr 2021 12:21:01 +1000
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Daniel Axtens <dja@axtens.net> writes:

> Hi Lakshmi,
>
>> On 4/15/21 12:14 PM, Lakshmi Ramasubramanian wrote:
>>
>> Sorry - missed copying device-tree and powerpc mailing lists.
>>
>>> There are a few "goto out;" statements before the local variable "fdt"
>>> is initialized through the call to of_kexec_alloc_and_setup_fdt() in
>>> elf64_load(). This will result in an uninitialized "fdt" being passed
>>> to kvfree() in this function if there is an error before the call to
>>> of_kexec_alloc_and_setup_fdt().
>>> 
>>> Initialize the local variable "fdt" to NULL.
>>>
> I'm a huge fan of initialising local variables! But I'm struggling to
> find the code path that will lead to an uninit fdt being returned...

OK, so perhaps this was putting it too strongly. I have been bitten
by uninitialised things enough in C that I may have taken a slightly
overly-agressive view of fixing them in the source rather than the
compiler. I do think compiler-level mitigations are better, and I take
the point that we don't want to defeat compiler checking.

(Does anyone - and by anyone I mean any large distro - compile with
local variables inited by the compiler?)

I was reading the version in powerpc/next, clearly I should have looked
at linux-next. Having said that, I think I will leave the rest of the
bikeshedding to the rest of you, you all seem to have it in hand :)

Kind regards,
Daniel

>
> The out label reads in part:
>
> 	/* Make kimage_file_post_load_cleanup free the fdt buffer for us. */
> 	return ret ? ERR_PTR(ret) : fdt;
>
> As far as I can tell, any time we get a non-zero ret, we're going to
> return an error pointer rather than the uninitialised value...
>
> (btw, it does look like we might leak fdt if we have an error after we
> successfully kmalloc it.)
>
> Am I missing something? Can you link to the report for the kernel test
> robot or from Dan? 
>
> FWIW, I think it's worth including this patch _anyway_ because initing
> local variables is good practice, but I'm just not sure on the
> justification.
>
> Kind regards,
> Daniel
>
>>> Signed-off-by: Lakshmi Ramasubramanian <nramas@linux.microsoft.com>
>>> Reported-by: kernel test robot <lkp@intel.com>
>>> Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
>>> ---
>>>   arch/powerpc/kexec/elf_64.c | 2 +-
>>>   1 file changed, 1 insertion(+), 1 deletion(-)
>>> 
>>> diff --git a/arch/powerpc/kexec/elf_64.c b/arch/powerpc/kexec/elf_64.c
>>> index 5a569bb51349..0051440c1f77 100644
>>> --- a/arch/powerpc/kexec/elf_64.c
>>> +++ b/arch/powerpc/kexec/elf_64.c
>>> @@ -32,7 +32,7 @@ static void *elf64_load(struct kimage *image, char *kernel_buf,
>>>   	int ret;
>>>   	unsigned long kernel_load_addr;
>>>   	unsigned long initrd_load_addr = 0, fdt_load_addr;
>>> -	void *fdt;
>>> +	void *fdt = NULL;
>>>   	const void *slave_code;
>>>   	struct elfhdr ehdr;
>>>   	char *modified_cmdline = NULL;
>>> 
>>
>> thanks,
>>   -lakshmi