Subject: Re: [PATCH] PCI: Fix endianness issues in pci_bus_read_config()

From: Bjorn Helgaas <[email-masked]>
Date: Thu, 31 Jul 2025 13:39:44 -0500

[+cc Arnd]

On Thu, Jul 31, 2025 at 07:38:58PM +0200, Gerd Bayer wrote:
> Simple pointer-casts to map byte and word reads from PCI config space
> into dwords (i.e. u32) produce unintended results on big-endian systems.
> Add the necessary adjustments under compile-time switch
> CONFIG_CPU_BIG_ENDIAN.
> 
> pci_bus_read_config() was just introduced with
> https://lore.kernel.org/all/[email-masked]/
> 
> Signed-off-by: Gerd Bayer <[email-masked]>
> ---
> 
> Hi Hans, hi Bjorn,
> 
> Sorry to spill this endianness aware code into drivers/pci, feel free to
> suggest a cleaner approach. This has fixed the issues seen on s390 systems
> Otherwise it is just compile-tested for x86 and arm64.
> 
> Since this is still sitting in the a pull-request for upstream, I'm not sure if this
> warrants a Fixes: tag.
> 
> Thanks,
> Gerd
> ---
>  drivers/pci/access.c | 25 +++++++++++++++++--------
>  1 file changed, 17 insertions(+), 8 deletions(-)
> 
> diff --git a/drivers/pci/access.c b/drivers/pci/access.c
> index ba66f55d2524..77a73b772a28 100644
> --- a/drivers/pci/access.c
> +++ b/drivers/pci/access.c
> @@ -89,15 +89,24 @@ int pci_bus_read_config(void *priv, unsigned int devfn, int where, u32 size,
>  			u32 *val)
>  {
>  	struct pci_bus *bus = priv;
> +	int rc;
>  
> -	if (size == 1)
> -		return pci_bus_read_config_byte(bus, devfn, where, (u8 *)val);
> -	else if (size == 2)
> -		return pci_bus_read_config_word(bus, devfn, where, (u16 *)val);
> -	else if (size == 4)
> -		return pci_bus_read_config_dword(bus, devfn, where, val);
> -	else
> -		return PCIBIOS_BAD_REGISTER_NUMBER;
> +	if (size == 1) {
> +		rc = pci_bus_read_config_byte(bus, devfn, where, (u8 *)val);
> +#if (IS_ENABLED(CONFIG_CPU_BIG_ENDIAN))
> +		*val = ((*val >> 24) & 0xff);
> +#endif

Yeah, this is all pretty ugly.  Obviously the previous code in
__pci_find_next_cap_ttl() didn't need this.  My guess is that was
because the destination for the read data was always the correct type
(u8/u16/u32), but here we always use a u32 and cast it to the
appropriate type.  Maybe we can use the correct types here instead of
the casts?

> +	} else if (size == 2) {
> +		rc = pci_bus_read_config_word(bus, devfn, where, (u16 *)val);
> +#if (IS_ENABLED(CONFIG_CPU_BIG_ENDIAN))
> +		*val = ((*val >> 16) & 0xffff);
> +#endif
> +	} else if (size == 4) {
> +		rc = pci_bus_read_config_dword(bus, devfn, where, val);
> +	} else {
> +		rc =  PCIBIOS_BAD_REGISTER_NUMBER;
> +	}
> +	return rc;
>  }
>  
>  int pci_generic_config_read(struct pci_bus *bus, unsigned int devfn,
> -- 
> 2.48.1
>