On Mon, Sep 08, 2025 at 03:53:49PM -0500, Bjorn Helgaas wrote:
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.
I had it planned for v6.18.
Maybe this should go in v6.17 regardless, to avoid a warning regression between this patch and cbc654d18d37?
Sure, or I could take it as part of the ffs series?
-Kees