linux-kernel - Re: [PATCH] ARM: cns3xxx: pci: avoid potential stack overflow

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Date:	Wed, 27 May 2015 08:45:38 +0200
From:	khalasa@...p.pl (Krzysztof Hałasa)
To:	Arnd Bergmann <arnd@...db.de>
Cc:	linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] ARM: cns3xxx: pci: avoid potential stack overflow

Arnd Bergmann <arnd@...db.de> writes:

> The cns3xxx_pcie_hw_init function uses excessive kernel
> stack space because of a hack that puts a fake struct
> pci_sys_data and struct pci_bus on the stack in order to
> call the generic pci_bus_read_config accessors, which causes
> a warning in ARM allmodconfig builds:
>
> arch/arm/mach-cns3xxx/pcie.c:266:1: warning: the frame size of 1080 bytes is larger than 1024 bytes
>
> This rewrites the code in question to use a private
> implementation of the config space access for the same
> purpose, getting rid of the local variables and the
> warning in the process. As part of this, we have to
> use an open-coded version of pci_bus_find_capability(),
> which unfortunately complicates the implementation.

Wouldn't it be better to simply use static structs for this purpose?
The hack isn't pretty, though.

> +	regs = cnspci->cfg0_regs + (PCI_DEVFN(1, 0) << 12);
> +

Some comment about would be helpful. Such as this:


> -	bus.number = 1; /* directly connected PCIe device */
                        ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

> +	pos = cns3xxx_pci_raw_read_config(regs, PCI_CAPABILITY_LIST, 1);
> +	while (cns3xxx_pci_raw_read_config(regs, pos, 1) != PCI_CAP_ID_EXP)
> +		pos = cns3xxx_pci_raw_read_config(regs, pos + PCI_CAP_LIST_NEXT, 1);
> +

I wonder if this can fail (i.e., no PCI_CAP_ID_EXP capability).

> +	dc = cns3xxx_pci_raw_read_config(regs, pos + PCI_EXP_DEVCTL, 2);
> +	dc &= ~(0x3 << 12);	/* Clear Device Control Register [14:12] */
> +	cns3xxx_pci_raw_write_config(regs, pos + PCI_EXP_DEVCTL, 2, dc);
> +	dc = cns3xxx_pci_raw_read_config(regs, pos + PCI_EXP_DEVCTL, 2);
> +	if (!(dc & (0x3 << 12)))
> +		pr_info("PCIe: Set Device Max_Read_Request_Size to 128 byte\n");
> +

This seems to revert 367dc4b75f4349d5363bc3ebdc030939db944786. Why do
you want to do it?
-- 
Krzysztof Halasa

Research Institute for Automation and Measurements PIAP
Al. Jerozolimskie 202, 02-486 Warsaw, Poland
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