[PATCH] powerpc: Initialize local variable fdt to NULL in elf64_load()

[Christophe Leroy]

Le 16/04/2021 à 08:44, Daniel Axtens a écrit :
> 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...
> 
> 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...

I don't think GCC is smart enough to detect that.

> 
> (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.

I don't think local systematically initing local variables is a good practice at all, as it leads to 
bugs where you get a wrong value because of pathes where you forgot to set the correct value.

If you don't init local variable at declaration and forget to set it in some pathes, the compiler 
will detect it and warn you.
If you init the local variable with an arbitrary value at declaration and forget to set it later in 
some pathes, the compiler won't be able to detect it and you will go with the wrong value.

Christophe

> 
> Kind regards,
> Daniel
> 
>>> Signed-off-by: Lakshmi Ramasubramanian <nramas at linux.microsoft.com>
>>> Reported-by: kernel test robot <lkp at intel.com>
>>> Reported-by: Dan Carpenter <dan.carpenter at 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