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Message-ID: <20250731183944.GA3424583@bhelgaas>
Date: Thu, 31 Jul 2025 13:39:44 -0500
From: Bjorn Helgaas <helgaas@...nel.org>
To: Gerd Bayer <gbayer@...ux.ibm.com>
Cc: Hans Zhang <18255117159@....com>, bhelgaas@...gle.com,
agordeev@...ux.ibm.com, borntraeger@...ux.ibm.com,
ilpo.jarvinen@...ux.intel.com, jingoohan1@...il.com,
kwilczynski@...nel.org, linux-kernel@...r.kernel.org,
linux-s390@...r.kernel.org, linux-next@...r.kernel.org,
linux-pci@...r.kernel.org, lpieralisi@...nel.org, mani@...nel.org,
robh@...nel.org, schnelle@...ux.ibm.com,
Arnd Bergmann <arnd@...nel.org>
Subject: Re: [PATCH] PCI: Fix endianness issues in pci_bus_read_config()
[+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/20250716161203.83823-2-18255117159@163.com/
>
> Signed-off-by: Gerd Bayer <gbayer@...ux.ibm.com>
> ---
>
> 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
>
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