modversions: redefine kcrctab entries as 32-bit values
From: Ard Biesheuvel <hidden>
Date: 2017-02-03 08:31:52
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linux-arch, linuxppc-dev, lkml
On 3 February 2017 at 05:09, Jessica Yu [off-list ref] wrote:
quoted hunk ↗ jump to hunk
+++ Jessica Yu [02/02/17 22:54 -0500]:quoted
+++ Ard Biesheuvel [24/01/17 16:16 +0000]:quoted
This v4 is a followup to [0] 'modversions: redefine kcrctab entries as relative CRC pointers', but since relative CRC pointers do not work in modules, and are actually only needed by powerpc with CONFIG_RELOCATABLE=y, I have made it a Kconfig selectable feature instead. Patch #1 introduces the MODULE_REL_CRCS Kconfig symbol, and adds the kbuild handling of it, i.e., modpost, genksyms and kallsyms. Patch #2 switches all architectures to 32-bit CRC entries in kcrctab, where all architectures except powerpc with CONFIG_RELOCATABLE=y use absolute ELF symbol references as before. v4: make relative CRCs kconfig selectable use absolute CRC symbols in modules regardless of kconfig selection split into two patchesThis asymmetry threw me off a bit, especially the Kconfig naming (only vmlinux crcs get the relative offsets, and only on powerpc atm, but all modules keep the absolute syms, but it is called MODULE_REL_CRCS...), if we keep this asymmetric crc treatment, it would be really nice to note this discrepancy this somewhere, perhaps in the Kconfig, to keep our heads from spinning :-) I'm still catching up on the previous discussion threads, but can you explain a bit more why you switched away from full blown relative crcs from your last patchset [1]? I had lightly tested your v3 on ppc64le previously, and relative offsets with modules seemed to worked very well. I'm probably missing something very obvious.Ah, I just saw your other comment about other arches not having support for the rel32 offsets :-/ The asymmetry still bothers me though. Can't we have something that just switches relative crcs on and off, that applies to *both* vmlinux and modules? Then we can get rid of the crc_owner check in check_version() and just have something like: if (IS_ENABLED(CONFIG_RELATIVE_CRCS)) crcval = resolve_crc(crc); Also we could get rid of the '&& !defined(MODULE)' checks scattered in export.h. Then the arches that want relative crcs and that *do* have rel32 relocation support can turn relative crcs on, and powerpc can enable it, right?
Indeed. My reasoning was that, given that we need to support both absolute and relative CRCs anyway, it would make sense for modules to keep using absolute ones regardless of what vmlinux is using, but it does obfuscate the code a bit.
Would that work or is there another reason this won't work with modules (assuming that the arches that select this option support the relative offsets)?
PowerPC with CONFIG_RELOCATABLE=y is really the only arch that requires this atm, and it supports REL32 relocations just fine, both for 32-bit and 64-bit. I will respin the patches with the above modification.