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

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Message-ID: <922bc558-4d5e-4e8d-bf9e-99fe0babfa5d@gmail.com>
Date: Thu, 13 Feb 2025 16:18:58 +0530
From: Purva Yeshi <purvayeshi550@...il.com>
To: Keith Busch <kbusch@...nel.org>, Bjorn Helgaas <helgaas@...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 02:48, Keith Busch wrote:
> On Tue, Feb 11, 2025 at 03:02:35PM -0600, Bjorn Helgaas wrote:
>> 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.
> 
> The original code is actually correct despite using a flexible array of
> a struct that contains a flexible array. That arrangement just means you
> can't index into it, but the code is only doing pointer arithmetic, so
> should be fine.
> 
> With this struct:
> 
> struct pci_saved_state {
>   	u32 config_space[16];
> 	struct pci_cap_saved_data cap[];
> };
> 
> Accessing "cap" field returns the address right after the config_space[]
> member. When it's changed to a pointer type, though, it needs to be
> initialized to *something* but the code doesn't do that. The code just
> expects the cap to follow right after the config.
> 
> Anyway, to silence the warning we can just make it an anonymous member
> and add 1 to the state to get to the same place in memory as before.
> 
> ---
> diff --git a/drivers/pci/pci.c b/drivers/pci/pci.c
> index 869d204a70a37..e562037644fd0 100644
> --- a/drivers/pci/pci.c
> +++ b/drivers/pci/pci.c
> @@ -1929,7 +1929,6 @@ EXPORT_SYMBOL(pci_restore_state);
>   
>   struct pci_saved_state {
>   	u32 config_space[16];
> -	struct pci_cap_saved_data cap[];
>   };
>   
>   /**
> @@ -1961,7 +1960,7 @@ struct pci_saved_state *pci_store_saved_state(struct pci_dev *dev)
>   	memcpy(state->config_space, dev->saved_config_space,
>   	       sizeof(state->config_space));
>   
> -	cap = state->cap;
> +	cap = (void *)(state + 1);
>   	hlist_for_each_entry(tmp, &dev->saved_cap_space, next) {
>   		size_t len = sizeof(struct pci_cap_saved_data) + tmp->cap.size;
>   		memcpy(cap, &tmp->cap, len);
> @@ -1991,7 +1990,7 @@ int pci_load_saved_state(struct pci_dev *dev,
>   	memcpy(dev->saved_config_space, state->config_space,
>   	       sizeof(state->config_space));
>   
> -	cap = state->cap;
> +	cap = (void *)(state + 1);
>   	while (cap->size) {
>   		struct pci_cap_saved_state *tmp;
>   
> --

Thanks for the clarification. I now see that the original use of a 
flexible array inside 'pci_saved_state' is correct. Would you like me to 
resend the patch with the modifications you suggested?