Re: [PATCH] drivers/pci: Fix FIELD_PREP compilation error with gcc-8

[Kees Cook <kees@kernel.org>]

On Thu, Aug 28, 2025 at 12:12:37PM +0200, Anders Roxell wrote:
> Commit cbc654d18d37 ("bitops: Add __attribute_const__ to generic
> ffs()-family implementations") causes a compilation failure on ARM
> footbridge_defconfig with gcc-8:
> 
>   FIELD_PREP: value too large for the field
> 
> The error occurs in pcie_set_readrq() at:
>   v = FIELD_PREP(PCI_EXP_DEVCTL_READRQ, ffs(rq) - 8);
> 
> With __attribute_const__, gcc-8 now performs wrong compile-time
> validation in FIELD_PREP and cannot guarantee that ffs(rq) - 8 will
> always produce values that fit in the 3-bit PCI_EXP_DEVCTL_READRQ field.

Thanks for examining this! It seems rather alarming -- why did it
work before?

> Avoid FIELD_PREP entirely by using direct bit manipulation. Replace
> FIELD_PREP(PCI_EXP_DEVCTL_READRQ, ffs(rq) - 8) with the equivalent
> manual bit operations: ((ffs(rq) - 8) << 12) & PCI_EXP_DEVCTL_READRQ.
> 
> This bypasses the compile-time validation while maintaining identical
> runtime behavior and functionality.

Did you dig into why this happened? It seems like a fragile situation,
so I'm worried we'll see more of these pop up.

> Fixes: cbc654d18d37 ("bitops: Add __attribute_const__ to generic ffs()-family implementations")
> Reported-by: Linux Kernel Functional Testing <lkft@linaro.org>
> Closes: https://lore.kernel.org/linux-pci/CA+G9fYuysVr6qT8bjF6f08WLyCJRG7aXAeSd2F7=zTaHHd7L+Q@mail.gmail.com/T/#u
> Signed-off-by: Anders Roxell <anders.roxell@linaro.org>
> ---
>  drivers/pci/pci.c | 3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/pci/pci.c b/drivers/pci/pci.c
> index e698278229f2..9f9607bd9f51 100644
> --- a/drivers/pci/pci.c
> +++ b/drivers/pci/pci.c
> @@ -5893,7 +5893,8 @@ int pcie_set_readrq(struct pci_dev *dev, int rq)
>  			rq = mps;
>  	}
>  
> -	v = FIELD_PREP(PCI_EXP_DEVCTL_READRQ, ffs(rq) - 8);
> +	/* Ideally we would used FIELD_PREP() but this is a work around for gcc-8 */
> +	v = ((ffs(rq) - 8) << 12) & PCI_EXP_DEVCTL_READRQ;
>  
>  	if (bridge->no_inc_mrrs) {
>  		int max_mrrs = pcie_get_readrq(dev);

If you're sure this is okay, I'll take it with the series, but I feel
like we should justify it better. :)

-Kees

-- 
Kees Cook