Messages in this thread | | | From | (Eric W. Biederman) | Subject | Re: [PATCH 02/02] Elf: Align elf notes properly | Date | Thu, 09 Nov 2006 22:09:26 -0700 |
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"Magnus Damm" <magnus.damm@gmail.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.
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