From mboxrd@z Thu Jan 1 00:00:00 1970 From: Ard Biesheuvel Subject: Re: [PATCH] efi/cper: Fix endianness of PCI class code Date: Fri, 26 May 2017 11:16:55 +0200 Message-ID: References: <20170506090755.GB19740@wunner.de> <20170510084143.GA16261@wunner.de> <20170525123047.GA4172@wunner.de> <20170525124445.GA4181@wunner.de> <20170525125650.GA4196@wunner.de> <20170526060843.GA4446@wunner.de> Mime-Version: 1.0 Content-Type: text/plain; charset="UTF-8" Return-path: In-Reply-To: <20170526060843.GA4446@wunner.de> Sender: linux-pci-owner@vger.kernel.org To: Lukas Wunner Cc: Arnd Bergmann , "linux-efi@vger.kernel.org" , Ashok Raj , linux-pci , Huang Ying List-Id: linux-efi@vger.kernel.org On 25 May 2017 at 23:08, Lukas Wunner wrote: > On Thu, May 25, 2017 at 06:07:35AM -0700, Ard Biesheuvel wrote: >> On 25 May 2017 at 05:56, Lukas Wunner wrote: >> > On Thu, May 25, 2017 at 05:47:59AM -0700, Ard Biesheuvel wrote: >> >> On 25 May 2017 at 05:44, Lukas Wunner wrote: >> >> > On Thu, May 25, 2017 at 05:36:01AM -0700, Ard Biesheuvel wrote: >> >> >> On 25 May 2017 at 05:30, Lukas Wunner wrote: >> >> >> > On Thu, May 11, 2017 at 03:06:42PM +0100, Ard Biesheuvel wrote: >> >> >> >> On 10 May 2017 at 09:41, Lukas Wunner wrote: >> >> >> >> > On Wed, May 10, 2017 at 09:03:11AM +0100, Ard Biesheuvel wrote: >> >> >> >> >> On 6 May 2017 at 10:07, Lukas Wunner wrote: >> >> >> >> >> > On Sat, May 06, 2017 at 08:46:07AM +0100, Ard Biesheuvel wrote: >> >> >> >> >> >> On 5 May 2017 at 19:38, Lukas Wunner wrote: >> >> >> >> >> >> > The CPER parser assumes that the class code is big endian, but at least >> >> >> >> >> >> > on this edk2-derived Intel Purley platform it's little endian: >> >> >> >> >> > [snip] >> >> >> >> >> >> > --- a/include/linux/cper.h >> >> >> >> >> >> > +++ b/include/linux/cper.h >> >> >> >> >> >> > @@ -416,7 +416,7 @@ struct cper_sec_pcie { >> >> >> >> >> >> > struct { >> >> >> >> >> >> > __u16 vendor_id; >> >> >> >> >> >> > __u16 device_id; >> >> >> >> >> >> > - __u8 class_code[3]; >> >> >> >> >> >> > + __u32 class_code:24; >> >> >> >> >> >> >> >> >> >> >> >> I'd like to avoid this change if we can. Couldn't we simply invert the >> >> >> >> >> >> order of p[] above? >> >> >> >> >> > >> >> >> >> >> > Hm, why would you like to avoid it? >> >> >> >> >> >> >> >> >> >> Because we shouldn't use bitfields in structs in code that should be >> >> >> >> >> portable across archs with different endiannesses. >> >> >> >> > >> >> >> >> > The CPER header is defined in the UEFI spec and UEFI mandates that the >> >> >> >> > arch is little endian (UEFI r2.6, sec. 2.3.5, 2.3.6). >> >> >> >> > >> >> >> >> >> >> >> >> No it does not mandate that at all. It mandates how the core should be >> >> >> >> configured when running in UEFI, but the OS can do anything it likes. >> >> >> >> >> >> >> >> We are still interested in adding limited UEFI support to big endian >> >> >> >> arm64 in the future (i.e., access to a limited set of firmware tables >> >> >> >> but no runtime services), and I am not going to merge anything that >> >> >> >> moves us away from that goal. >> >> >> >> >> >> >> >> > So your argument seems moot to me. Am I missing something? Do you >> >> >> >> > have another argument? >> >> >> >> > >> >> >> >> > Moreover, the vendor_id and device_id fields are little endian as well >> >> >> >> > (PCI r3.0, sec. 6.1), yet there are no provisions in our CPER parser in >> >> >> >> > drivers/firmware/efi/cper.c to convert them to the endianness of the host. >> >> >> >> > >> >> >> >> >> >> >> >> Indeed. I am aware we will need to add various endian-neutral >> >> >> >> accessors in the future. >> >> >> >> >> >> >> >> >> > The class_code element isn't >> >> >> >> >> > referenced anywhere else in the kernel and this isn't a uapi header, >> >> >> >> >> > so the change would only impact out-of-tree drivers. Not sure if >> >> >> >> >> > any exist which might be interested in CPER parsing. >> >> >> >> >> > >> >> >> >> >> >> >> >> >> >> The point is that the change in the struct definition is simply not >> >> >> >> >> necessary, given that inverting the order of p[] already achieves >> >> >> >> >> exactly what we want. >> >> >> >> > >> >> >> >> > It seems clumsy and unnecessary to me so I'd prefer the bitfield. >> >> >> >> > Please excuse my stubbornness. >> >> >> >> > >> >> >> >> >> >> >> >> Stubbornness alone is not going to convince me. What *could* convince >> >> >> >> me (although unlikely) is a quote from the C spec which explains why >> >> >> >> it is 100% legal to make assumptions about how bitfields are projected >> >> >> >> onto byte locations in memory. >> >> >> > >> >> >> > All structs in cper.h are declared "packed", so what you're asking for >> >> >> > isn't defined in the C spec but in the GCC documentation: >> >> >> > >> >> >> > "The packed attribute specifies that a variable or structure field >> >> >> > should have the smallest possible alignment -- one byte for a variable, >> >> >> > and one bit for a field, unless you specify a larger value with the >> >> >> > aligned attribute." >> >> >> > >> >> >> > So I maintain that the patch is fine, but you'll need to use le32_to_cpu(), >> >> >> > le16_to_cpu() etc both for the class_code changed by the patch as well as >> >> >> > all the other members of the struct not touched by the patch when adding >> >> >> > "endianness mixed mode" for aarch64. >> >> >> >> >> >> I'm not talking about the 'packed' attribute but about the fact that >> >> >> the C spec does not guarantee that bitfields are projected onto byte >> >> >> locations in memory in the way you expect. >> >> > >> >> > What relevance does that have as long as the header file uses a pragma >> >> > specific to gcc (or other compilers that are compatible to gcc with >> >> > respect to that pragma (such as clang)), and gcc guarantees the >> >> > correct layout regardless of endianness? >> >> >> >> The relevance is that we should not add GCC specific code because you >> >> think it looks prettier. >> > >> > The code already *is* gcc-specific. >> > >> >> The entire kernel is GCC specific. But that does not justify adding >> more GCC-isms throughout the code. > > How is the patch adding a GCC-ism? > Because you rely on behavior which is not defined by the C spec. > >> >> And where does GCC guarantee the correct layout? Did you find an >> >> unambiguous GCC documentation reference that explains how bitfields >> >> are mapped onto byte locations? >> > >> > See the excerpt I quoted above. >> > >> >> 'packed' has nothing to do with it. This is about bitfields in structs. > > 'packed' has *everything* to do with it. :-) > > The struct contains a *single* bitfield surrounded by non-bitfields. > If there were multiple consecutive bitfields, then yes, things wouldn't > be as clear. > > The bitfield as well as all surrounding non-bitfields have a size which > is a multiple of full bytes. And this is where the 'packed' attribute > comes into play, it guarantees that there's no padding as long as all > members of the struct are byte-aligned. > No, it does not guarantee that at all. Observed behavior != guarantee. 'Guarantee' implies that it is documented in a pertinent spec, and that we can file a bug with the GCC/Clang projects if the behavior changes at any point. Nobody is asking you to theorize and make inferences about how attribute X and behavior Y offer guarantee Z. All it takes is an unambiguous quote from the C spec that describes how the struct definition is mapped onto bits in memory. You have offered no such quote, for which I don't blame you because I am convinced that the C spec does not define this in sufficient detail. > >> >> > >> >> Or does 'guarantee' mean 'I tested it and it works'? >> > >> > I tested it with x86_64 (le) and ppc32 (be) and it works. >> > I don't have an aarch64 machine available here. >> > >> >> Good. > > Good to merge then? > No. For the last time, this patch will not be merged. The only approach that will be merged is keeping the char[3] array and inverting the order of the printk() arguments. -- Ard.