Re: [PATCH v10 04/10] kexec_file: Add support for purgatory built as PIE.

[Thiago Jung Bauermann]

Am Dienstag, 22. November 2016, 17:01:10 BRST schrieb Michael Ellerman:
> Thiago Jung Bauermann <bauerman@xxxxxxxxxxxxxxxxxx> writes:
> > Am Sonntag, 20. November 2016, 10:45:46 BRST schrieb Dave Young:
> >> On 11/10/16 at 01:27am, Thiago Jung Bauermann wrote:
> >> > powerpc's purgatory.ro has 12 relocation types when built as
> >> > a relocatable object. To implement support for them requires
> >> > arch_kexec_apply_relocations_add to duplicate a lot of code with
> >> > module_64.c:apply_relocate_add.
> >> > 
> >> > When built as a Position Independent Executable there are only 4
> >> > relocation types in purgatory.ro, so it becomes practical for the
> >> > powerpc
> >> > implementation of kexec_file to have its own relocation implementation.
> >> > 
> >> > Also, the purgatory is an executable and not an intermediary output
> >> > from
> >> > the compiler so it makes sense conceptually that it is easier to build
> >> > it as a PIE than as a partially linked object.
> >> > 
> >> > Apart from the greatly reduced number of relocations, there are two
> >> > differences between a relocatable object and a PIE:
> >> > 
> >> > 1. __kexec_load_purgatory needs to use the program headers rather than
> >> > the
> >> > 
> >> >    section headers to figure out how to load the binary.
> >> > 
> >> > 2. Symbol values are absolute addresses instead of relative to the
> >> > 
> >> >    start of the section.
> >> > 
> >> > This patch adds the support needed in generic code for the differences
> >> > above and allows powerpc to load and relocate a position independent
> >> > purgatory.
> >> 
> >> [snip]
> >> 
> >> The kexec-tools machine_apply_elf_rel is pretty simple for ppc64, it is
> >> not that complex. So could you look into simplify your kexec_file
> >> implementation?
> > 
> > I can try, but there is one fundamental issue here: powerpc
> > position-dependent code relies more on relocations than x86
> > position-dependent code does, so there's a limit to how simple it can be
> > made without switching to position- independent code. And it will always
> > be more involved than it is on x86.
> I think we need to go back to the drawing board on this one.
> 
> My hope was that building purgatory as PIE would reduce the amount of
> complexity, but instead it's just added more. Sorry for sending you in
> that direction.

It added complexity because in my series powerpc was using a PIE purgatory but 
x86 kept using a partially-linked object (because of the problem I mentioned I 
had when trying out a PIE x86 purgatory), so generic code needed two purgatory 
loaders.

I'll see if I can make the PIE x86 purgatory to work so that generic code can 
have only one loader implementation. Then it will indeed be simpler.


Am Dienstag, 22. November 2016, 14:16:22 BRST schrieb Dave Young:
> Hi Michael
> 
> On 11/22/16 at 05:01pm, Michael Ellerman wrote:
> > In general I dislike the level of complexity of the kexec-tools
> > purgatory, and in particular I'm not comfortable with things like:
> > 
> > diff --git a/arch/powerpc/purgatory/sha256.c
> > b/arch/powerpc/purgatory/sha256.c new file mode 100644
> > index 000000000000..6abee1877d56
> > --- /dev/null
> > +++ b/arch/powerpc/purgatory/sha256.c
> > @@ -0,0 +1,6 @@
> > +#include "../boot/string.h"
> > +
> > +/* Avoid including x86's boot/string.h in sha256.c. */
> > +#define BOOT_STRING_H
> > +
> > +#include "../../x86/purgatory/sha256.c"
> 
> Agreed, include x86 code in powerpc looks bad
> 
> > I think the best way to get this over the line would be to take the
> > kexec-lite purgatory implementation and use that to begin with. I know
> > it doesn't have all the features of the kexec-tools version, but it
> > should work, and we can look at adding the extra features later.
> 
> Instead of adding other implementation, moving the purgatory sha256 code
> out of x86 sounds better so that we can reuse them cleanly..

Do you have a suggestion of where that code can live so that it can be shared 
between purgatories for different arches?

Do we need a purgatory with generic and arch-specific code like in kexec-
tools?

-- 
Thiago Jung Bauermann
IBM Linux Technology Center