Re: [PATCH] i2c: designware: Remove i2c_dw_remove_lock_support()

[Kees Cook <kees@kernel.org>]

On Mon, Oct 13, 2025 at 06:05:03PM -0700, Nathan Chancellor wrote:
> When building certain configurations with CONFIG_FINEIBT=y after
> commit 894af4a1cde6 ("objtool: Validate kCFI calls"), there is a
> warning due to an indirect call in dw_i2c_plat_remove():
> 
>   $ cat allno.config
>   CONFIG_ACPI=y
>   CONFIG_CFI=y
>   CONFIG_COMMON_CLK=y
>   CONFIG_CPU_MITIGATIONS=y
>   CONFIG_I2C=y
>   CONFIG_I2C_DESIGNWARE_BAYTRAIL=y
>   CONFIG_I2C_DESIGNWARE_CORE=y
>   CONFIG_I2C_DESIGNWARE_PLATFORM=y
>   CONFIG_IOSF_MBI=y
>   CONFIG_MITIGATION_RETPOLINE=y
>   CONFIG_MODULES=y
>   CONFIG_PCI=y
>   CONFIG_X86_KERNEL_IBT=y
> 
>   $ make -skj"$(nproc)" ARCH=x86_64 LLVM=1 clean allnoconfig vmlinux
>   vmlinux.o: warning: objtool: dw_i2c_plat_remove+0x3c: no-cfi indirect call!
> 
> With this configuration, i2c_dw_semaphore_cb_table has the BAYTRAIL
> member and the sentinel (i.e., 2 members), both of which have an
> implicit
> 
>   .remove = NULL,
> 
> so Clang effectively turns i2c_dw_remove_lock_support(), which is later
> inlined into dw_i2c_plat_remove(), into:
> 
>   static void i2c_dw_remove_lock_support(struct dw_i2c_dev *dev)
>   {
>       if (dev->semaphore_idx > 2)
>           (*NULL)(dev):
>   }
> 
> which is not necessarily problematic from a logic perspective (as the
> code was not bounds checking semaphore_idx so an out of bounds index
> could already crash) but objtool's new __nocfi indirect call checking
> trips over Clang dropping the kCFI setup from a known NULL indirect
> call.
> 
> While it would be possible to fix this by transforming the initial check
> into
> 
>   if (dev->semaphore_idx < 0 || dev->semaphore_idx >= ARRAY_SIZE(i2c_dw_semaphore_cb_table))
> 
> the remove member is unused after commit 440da737cf8d ("i2c: designware:
> Use PCI PSP driver for communication"), so i2c_dw_remove_lock_support()
> can be removed altogether, as it will never actually do anything.
> 
> Closes: https://github.com/ClangBuiltLinux/linux/issues/2133
> Signed-off-by: Nathan Chancellor <nathan@kernel.org>

Thanks for the analysis!

Reviewed-by: Kees Cook <kees@kernel.org>

-- 
Kees Cook