Re: [PATCH] PCI: Test for bit underflow in pcie_set_readrq()

[Kees Cook <kees@kernel.org>]

On Fri, Sep 05, 2025 at 10:25:49AM +0200, Arnd Bergmann wrote:
> On Fri, Sep 5, 2025, at 07:28, 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")
> > Reported-by: Linux Kernel Functional Testing <lkft@linaro.org>
> > Closes: 
> > https://lore.kernel.org/linux-pci/CA+G9fYuysVr6qT8bjF6f08WLyCJRG7aXAeSd2F7=zTaHHd7L+Q@mail.gmail.com/
> > Signed-off-by: Kees Cook <kees@kernel.org>
> 
> Acked-by: Arnd Bergmann <arnd@arndb.de>
> 
> This looks good to me individually, however I have now tried to
> do more randconfig tests with the __attribute_const change
> and gcc-8.5.0, and so far found two other files with the
> same issue:
> 
> In file included from <command-line>:
> In function 'mtk_dai_etdm_out_configure.constprop',
>     inlined from 'mtk_dai_etdm_configure' at sound/soc/mediatek/mt8188/mt8188-dai-etdm.c:2168:3:
> include/linux/compiler_types.h:575:38: error: call to '__compiletime_assert_416' declared with attribute error: FIELD_PREP: value too large for the field
> sound/soc/mediatek/mt8188/mt8188-dai-etdm.c:2065:10: note: in expansion of macro 'FIELD_PREP'
>    val |= FIELD_PREP(ETDM_OUT_CON4_FS_MASK, get_etdm_fs_timing(rate));
> sound/soc/mediatek/mt8188/mt8188-dai-etdm.c:1971:10: note: in expansion of macro 'FIELD_PREP'
>    val |= FIELD_PREP(ETDM_IN_CON3_FS_MASK, get_etdm_fs_timing(rate));
> 
> drivers/mmc/host/meson-gx-mmc.c: In function 'meson_mmc_start_cmd':
> drivers/mmc/host/meson-gx-mmc.c:811:14: note: in expansion of macro 'FIELD_PREP'
>    cmd_cfg |= FIELD_PREP(CMD_CFG_TIMEOUT_MASK,

I can't see how these are related to ffs()...

> This is fairly rare, but over time there are likely going to be
> others like them. I see three possible ways forward here:
> 
> a) fix them individually as we run into them, hoping for the best

I think they are valid warnings, so my instinct would be to fix them as
they appear. (e.g. "0 - 8" isn't something FIELD_PREP can do anything
about.)

-Kees

-- 
Kees Cook