linux-kernel - [PATCH v4 1/3] kernel/modules: Add REL24 relocation support of livepatch symbols

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Date:   Tue, 14 Nov 2017 04:29:08 -0500
From:   Kamalesh Babulal <kamalesh@...ux.vnet.ibm.com>
To:     Michael Ellerman <mpe@...erman.id.au>
Cc:     Kamalesh Babulal <kamalesh@...ux.vnet.ibm.com>,
        linux-kernel@...r.kernel.org, linuxppc-dev@...ts.ozlabs.org,
        live-patching@...r.kernel.org,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        Balbir Singh <bsingharora@...il.com>,
        "Naveen N . Rao" <naveen.n.rao@...ux.vnet.ibm.com>,
        Jessica Yu <jeyu@...nel.org>,
        Ananth N Mavinakayanahalli <ananth@...ux.vnet.ibm.com>,
        Aravinda Prasad <aravinda@...ux.vnet.ibm.com>,
        Torsten Duwe <duwe@....de>
Subject: [PATCH v4 1/3] kernel/modules: Add REL24 relocation support of livepatch symbols

Livepatch re-uses module loader function apply_relocate_add() to write
relocations, instead of managing them by arch-dependent
klp_write_module_reloc() function.

apply_relocate_add() doesn't understand livepatch symbols (marked with
SHN_LIVEPATCH symbol section index) and assumes them to be local symbols
by default for R_PPC64_REL24 relocation type. It fails with an error,
when trying to calculate offset with local_entry_offset():

 module_64: kpatch_meminfo: REL24 -1152921504897399800 out of range!

Whereas livepatch symbols are essentially SHN_UNDEF, should be
called via stub used for global calls. This issue can be fixed by
teaching apply_relocate_add() to handle both SHN_UNDEF/SHN_LIVEPATCH
symbols via the same stub. This patch extends SHN_UNDEF code to handle
livepatch symbols too.

Signed-off-by: Kamalesh Babulal <kamalesh@...ux.vnet.ibm.com>
CC: Balbir Singh <bsingharora@...il.com>
Cc: Naveen N. Rao <naveen.n.rao@...ux.vnet.ibm.com>
Cc: Josh Poimboeuf <jpoimboe@...hat.com>
Cc: Jessica Yu <jeyu@...nel.org>
Cc: Ananth N Mavinakayanahalli <ananth@...ux.vnet.ibm.com>
Cc: Aravinda Prasad <aravinda@...ux.vnet.ibm.com>
Cc: Torsten Duwe <duwe@....de>
---
 arch/powerpc/kernel/module_64.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/arch/powerpc/kernel/module_64.c b/arch/powerpc/kernel/module_64.c
index 0b0f896..39b01fd 100644
--- a/arch/powerpc/kernel/module_64.c
+++ b/arch/powerpc/kernel/module_64.c
@@ -613,7 +613,8 @@ int apply_relocate_add(Elf64_Shdr *sechdrs,

 		case R_PPC_REL24:
 			/* FIXME: Handle weak symbols here --RR */
-			if (sym->st_shndx == SHN_UNDEF) {
+			if (sym->st_shndx == SHN_UNDEF ||
+			    sym->st_shndx == SHN_LIVEPATCH) {
 				/* External: go via stub */
 				value = stub_for_addr(sechdrs, value, me);
 				if (!value)
-- 
2.9.3