Am Dienstag, 22. November 2016, 17:01:10 BRST schrieb Michael Ellerman:

Thiago Jung Bauermann [off-list ref] writes:

quoted

Am Sonntag, 20. November 2016, 10:45:46 BRST schrieb Dave Young:

quoted

On 11/10/16 at 01:27am, Thiago Jung Bauermann wrote:

quoted

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:

quoted

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

quoted

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


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