linux-kernel - Re: [PATCH] PCI: Fix endianness issues in pci_bus_read

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Message-ID: <d2240ab0-5d91-4b41-945f-e29b40f7b7f4@163.com>
Date: Sat, 2 Aug 2025 23:23:34 +0800
From: Hans Zhang <18255117159@....com>
To: Keith Busch <kbusch@...nel.org>
Cc: Gerd Bayer <gbayer@...ux.ibm.com>, Manivannan Sadhasivam
 <mani@...nel.org>, Hans Zhang <hans.zhang@...tech.com>,
 Arnd Bergmann <arnd@...nel.org>, Bjorn Helgaas <helgaas@...nel.org>,
 bhelgaas@...gle.com, Alexander Gordeev <agordeev@...ux.ibm.com>,
 Christian Borntraeger <borntraeger@...ux.ibm.com>,
 Ilpo Järvinen <ilpo.jarvinen@...ux.intel.com>,
 jingoohan1@...il.com, Krzysztof Wilczy´nski
 <kwilczynski@...nel.org>, linux-kernel@...r.kernel.org,
 linux-s390@...r.kernel.org, linux-next <linux-next@...r.kernel.org>,
 linux-pci@...r.kernel.org, Lorenzo Pieralisi <lpieralisi@...nel.org>,
 Rob Herring <robh@...nel.org>, Niklas Schnelle <schnelle@...ux.ibm.com>,
 geert@...ux-m68k.org
Subject: Re: [PATCH] PCI: Fix endianness issues in pci_bus_read_config()



On 2025/8/2 02:08, Keith Busch wrote:
> On Sat, Aug 02, 2025 at 12:54:27AM +0800, Hans Zhang wrote:
>> As I mentioned in my reply to Mani's email, the data ultimately read here is
>> also a forced type conversion.
>>
>> #define PCI_OP_READ(size, type, len) \
>> int noinline pci_bus_read_config_##size \
>> 	(struct pci_bus *bus, unsigned int devfn, int pos, type *value)	\
>> {									\
>> 	unsigned long flags;						\
>> 	u32 data = 0;							\
>> 	int res;							\
>> 									\
>> 	if (PCI_##size##_BAD)						\
>> 		return PCIBIOS_BAD_REGISTER_NUMBER;			\
>> 									\
>> 	pci_lock_config(flags);						\
>> 	res = bus->ops->read(bus, devfn, pos, len, &data);		\
>> 	if (res)							\
>> 		PCI_SET_ERROR_RESPONSE(value);				\
>> 	else								\
>> 		*value = (type)data;					\
>> 	pci_unlock_config(flags);					\
>> 									\
>> 	return res;							\
>> }
>>
>> And this function. Could it be that I misunderstood something?
> 
> The above macro retains the caller's type for "value". If the caller
> passes a "u8 *", the value is deferenced as a u8.

Dear Keith,

In this macro definition, bus->ops->read needs to ensure the byte order 
of the read, as Lukas mentioned; otherwise, there is also a big-endian 
issue at this location.

> 
> The function below promotes everything to a u32 pointer and deferences
> it as such regardless of what type the user passed in.

I searched and learned that readb/readw/readl automatically handle byte 
order, so there is no big-endian order issue.

>   
>> int pci_generic_config_read(struct pci_bus *bus, unsigned int devfn,
>> 			    int where, int size, u32 *val)
>> {
>> 	void __iomem *addr;
>>
>> 	addr = bus->ops->map_bus(bus, devfn, where);
>> 	if (!addr)
>> 		return PCIBIOS_DEVICE_NOT_FOUND;
>>
>> 	if (size == 1)
>> 		*val = readb(addr);
>> 	else if (size == 2)
>> 		*val = readw(addr);
>> 	else
>> 		*val = readl(addr);
>>
>> 	return PCIBIOS_SUCCESSFUL;
>> }


Best regards,
Hans