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@xxxxxxxxxx>
> Closes: https://lore.kernel.org/linux-pci/CA+G9fYuysVr6qT8bjF6f08WLyCJRG7aXAeSd2F7=zTaHHd7L+Q@xxxxxxxxxxxxxx/T/#u
> Signed-off-by: Anders Roxell <anders.roxell@xxxxxxxxxx>
> ---
> 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
