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Date: Thu, 09 Nov 2006 22:09:26 -0700 From: ebiederm@...ssion.com (Eric W. Biederman) To: "Magnus Damm" <magnus.damm@...il.com> Cc: "Magnus Damm" <magnus@...inux.co.jp>, linux-kernel@...r.kernel.org, "Vivek Goyal" <vgoyal@...ibm.com>, "Andi Kleen" <ak@....de>, fastboot@...ts.osdl.org, Horms <horms@...ge.net.au>, "Dave Anderson" <anderson@...hat.com> Subject: Re: [PATCH 02/02] Elf: Align elf notes properly "Magnus Damm" <magnus.damm@...il.com> writes: > I'm not sure you see all my points. The important parts are the > offsets - offset 0 and offset N2 in the description above. The should > be aligned somehow. Exactly how to align them depends on if the 64-bit > spec is valid or not. > > My points are: > > - Some kdump code rounds up the size of "elf note header" today. This > is unneccessary for 32 bit alignment and plain wrong for 64 bit > alignment. So I think that the code is strange and should be changed > regardless if the 64-bit spec is valid or not. Sure that is reasonable, if correct. > - Many implementations incorrectly calculate N2 as: roundup(sizeof(elf > note header)) + roundup(n_namesz). I am not certain that is incorrect. roundup(sizeof(elf note header), 4) + roundup(n_namesize, 4) will yield something that is properly 4 byte aligned. I do agree that implementation is not correct for 8 byte alignment. 8 byte alignment does not appear to be in widespread use in the wild. > - You say that the size of the notes do not vary and therefore this is > a non-issue. I agree that the size does not vary, but I believe that > the aligment _is_ an issue. One example is the N2 calculation above, > but more importantly the vmcore code that merges the elf note sections > into one. You know, if you have more than one cpu you will end up with > more than one crash note. And if you run Xen you will have even more > crash notes. Sure that is clearly an issue. > - On top of this I think it would be nice if all this code could be > unified to avoid code duplication. But we need to straighten out this > and agree on how the aligment should work before the code can be > merged into one implementation. Sure. To verify your claim that 8 byte alignment is correct I checked the core dump code in fs/binfmt_elf.c in the linux kernel. That always uses 4 byte alignment. Therefore it appears clear that only doing 4 byte alignment is not a local misreading of the spec, and is used in other implementations. If you can find an implementation that uses 8 byte alignment I am willing to consider it. The current situation is that the linux kernel generated application core dumps use 4 byte alignment so I expect that is what existing applications such as gdb expect. Therefore we use 4 byte alignment unless it can be shown that the linux core dumps are a fluke and should be fixed. Eric - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@...r.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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