Messages in this thread | | | Date | Fri, 10 Nov 2006 15:53:57 +0900 | From | "Magnus Damm" <> | Subject | Re: [PATCH 02/02] Elf: Align elf notes properly |
| |
On 11/10/06, Eric W. Biederman <ebiederm@xmission.com> wrote: > "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 are correct that it only matters if we are interested in 8 byte alignment. So it should be a non-issue for the 4-byte aligment case.
> > - 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.
Yes, fs/binfmt_elf.c is one of the files that my patch modifies. There are several elf note implementations in the kernel, all seem to use 4-byte aligment.
Implementations that use 8-byte alignment:
binutils-2.16.1/bfd/elf.c: elf_core_write_note() is using log_file_align which is set to 3 on some 64-bit platforms. 8-byte alignment in some cases.
binutils-2.16.1/binutils/readelf.c: process_corefile_note_segment() is always using 4-byte alignment though.
> 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.
Most applications probably expect 4-byte aligned data. OTOH, I just came across HP's ELF-64 Object File Format document. It says that 8-byte alignment should be used:
http://devresource.hp.com/drc/STK/docs/refs/elf-64-hp.pdf
So now we have two documents that say 8-byte alignment should be used.
> Therefore we use 4 byte alignment unless it can be shown that the > linux core dumps are a fluke and should be fixed.
Ok. Vivek, Dave, anyone? Comments?
/ magnus - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
| |