linux-kernel - Re: [PATCH] drivers: pci: Fix flexible array usage

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Message-ID: <274de7be-86f4-489a-befe-475ae4c51826@gmail.com>
Date: Thu, 13 Feb 2025 16:12:03 +0530
From: Purva Yeshi <purvayeshi550@...il.com>
To: Bjorn Helgaas <helgaas@...nel.org>, Keith Busch <kbusch@...nel.org>
Cc: bhelgaas@...gle.com, skhan@...uxfoundation.org,
 linux-pci@...r.kernel.org, linux-kernel@...r.kernel.org,
 Alex Williamson <alex.williamson@...hat.com>
Subject: Re: [PATCH] drivers: pci: Fix flexible array usage

On 12/02/25 02:32, Bjorn Helgaas wrote:
> On Mon, Feb 10, 2025 at 04:03:26PM -0700, Keith Busch wrote:
>> On Mon, Feb 10, 2025 at 06:57:40PM +0530, Purva Yeshi wrote:
>>> Fix warning detected by smatch tool:
>>> Array of flexible structure occurs in 'pci_saved_state' struct
>>>
>>> The warning occurs because struct pci_saved_state contains struct
>>> pci_cap_saved_data cap[], where cap[] has a flexible array member (data[]).
>>> Arrays of structures with flexible members are not allowed, leading to this
>>> warning.
>>>
>>> Replaced cap[] with a pointer (*cap), allowing dynamic memory allocation
>>> instead of embedding an invalid array of flexible structures.
>>>
>>> Signed-off-by: Purva Yeshi <purvayeshi550@...il.com>
>>> ---
>>>   drivers/pci/pci.c | 2 +-
>>>   1 file changed, 1 insertion(+), 1 deletion(-)
>>>
>>> diff --git a/drivers/pci/pci.c b/drivers/pci/pci.c
>>> index 869d204a7..648a080ef 100644
>>> --- a/drivers/pci/pci.c
>>> +++ b/drivers/pci/pci.c
>>> @@ -1929,7 +1929,7 @@ EXPORT_SYMBOL(pci_restore_state);
>>>   
>>>   struct pci_saved_state {
>>>   	u32 config_space[16];
>>> -	struct pci_cap_saved_data cap[];
>>> +	struct pci_cap_saved_data *cap;
>>>   };
>>
>> I don't think this is right. Previously the space for "cap" was
>> allocated at the end of the pci_saved_state, but now it's just an
>> uninitialized pointer.
> 
> Thanks, I think you're right.  Dropped pending fix or better
> explanation.
> 
> This is kind of a complicated data structure.  IIUC, a struct
> pci_saved_state is allocated only in pci_store_saved_state(), where
> the size is determined by the sum of the sizes of all the entries in
> the dev->saved_cap_space list.
> 
> The pci_saved_state is filled by copying from entries in the
> dev->saved_cap_space list.  The entries need not be all the same size
> because we copy each entry manually based on its size.
> 
> So cap[] is really just the base of this buffer of variable-sized
> entries.  Maybe "struct pci_cap_saved_data cap[]" is not the best
> representation of this, but *cap (a pointer) doesn't seem better.
> 
> Bjorn

Thanks for the explanation. The primary goal of the patch was to address 
the Smatch warning regarding the flexible array member inside 
'pci_cap_saved_data'. However, from the explanation, I now got that the 
current approach may not be ideal.