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

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Message-ID: <20250211210235.GA54524@bhelgaas>
Date: Tue, 11 Feb 2025 15:02:35 -0600
From: Bjorn Helgaas <helgaas@...nel.org>
To: Keith Busch <kbusch@...nel.org>
Cc: Purva Yeshi <purvayeshi550@...il.com>, 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 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