On 11/10/06, Eric W. Biederman <[EMAIL PROTECTED]> wrote: > "Magnus Damm" <[EMAIL PROTECTED]> 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 _______________________________________________ fastboot mailing list [email protected] https://lists.osdl.org/mailman/listinfo/fastboot
