On Thu, Sep 04, 2025 at 10:28:41PM -0700, Kees Cook wrote:
> After commit cbc654d18d37 ("bitops: Add __attribute_const__ to generic
> ffs()-family implementations"), which allows GCC's value range tracker
> to see past ffs(), GCC 8 on ARM thinks that it might be possible that
> "ffs(rq) - 8" used here:
>
> v = FIELD_PREP(PCI_EXP_DEVCTL_READRQ, ffs(rq) - 8);
>
> could wrap below 0, leading to a very large value, which would be out of
> range for the FIELD_PREP() usage:
>
> drivers/pci/pci.c: In function 'pcie_set_readrq':
> include/linux/compiler_types.h:572:38: error: call to
> '__compiletime_assert_471' declared with attribute error: FIELD_PREP: value
> too large for the field
> ...
> drivers/pci/pci.c:5896:6: note: in expansion of macro 'FIELD_PREP'
> v = FIELD_PREP(PCI_EXP_DEVCTL_READRQ, ffs(rq) - 8);
> ^~~~~~~~~~
>
> If the result of the ffs() is bounds checked before being used in
> FIELD_PREP(), the value tracker seems happy again. :)
>
> Fixes: cbc654d18d37 ("bitops: Add __attribute_const__ to generic ffs()-family
> implementations")
What's your plan for merging cbc654d18d37? I suppose it's intended
for v6.18? If it will appear in v6.17, let me know so I can merge
this for it as well.
Maybe this should go in v6.17 regardless, to avoid a warning
regression between this patch and cbc654d18d37?
> Reported-by: Linux Kernel Functional Testing <[email protected]>
> Closes:
> https://lore.kernel.org/linux-pci/CA+G9fYuysVr6qT8bjF6f08WLyCJRG7aXAeSd2F7=ztahhd7...@mail.gmail.com/
> Signed-off-by: Kees Cook <[email protected]>
> ---
> Cc: Bjorn Helgaas <[email protected]>
> Cc: Anders Roxell <[email protected]>
> Cc: Naresh Kamboju <[email protected]>
> Cc: [email protected]
> Cc: Linux Regressions <[email protected]>
> Cc: Arnd Bergmann <[email protected]>
> Cc: Dan Carpenter <[email protected]>
> Cc: Ben Copeland <[email protected]>
> Cc: <[email protected]>
> Cc: <[email protected]>
> Cc: <[email protected]>
> ---
> drivers/pci/pci.c | 6 +++++-
> 1 file changed, 5 insertions(+), 1 deletion(-)
>
> diff --git a/drivers/pci/pci.c b/drivers/pci/pci.c
> index b0f4d98036cd..005b92e6585e 100644
> --- a/drivers/pci/pci.c
> +++ b/drivers/pci/pci.c
> @@ -5932,6 +5932,7 @@ int pcie_set_readrq(struct pci_dev *dev, int rq)
> {
> u16 v;
> int ret;
> + unsigned int firstbit;
> struct pci_host_bridge *bridge = pci_find_host_bridge(dev->bus);
>
> if (rq < 128 || rq > 4096 || !is_power_of_2(rq))
> @@ -5949,7 +5950,10 @@ int pcie_set_readrq(struct pci_dev *dev, int rq)
> rq = mps;
> }
>
> - v = FIELD_PREP(PCI_EXP_DEVCTL_READRQ, ffs(rq) - 8);
> + firstbit = ffs(rq);
> + if (firstbit < 8)
> + return -EINVAL;
> + v = FIELD_PREP(PCI_EXP_DEVCTL_READRQ, firstbit - 8);
>
> if (bridge->no_inc_mrrs) {
> int max_mrrs = pcie_get_readrq(dev);
> --
> 2.34.1
>
