linux-kernel - [PATCH V2 2/4] powerpc: add helper to check if offset is within rel branch range

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Date:   Wed, 14 Dec 2016 21:48:30 +0530
From:   Anju T Sudhakar <anju@...ux.vnet.ibm.com>
To:     linux-kernel@...r.kernel.org, linuxppc-dev@...ts.ozlabs.org
Cc:     ananth@...ibm.com, naveen.n.rao@...ux.vnet.ibm.com,
        paulus@...ba.org, srikar@...ux.vnet.ibm.com,
        benh@...nel.crashing.org, mpe@...erman.id.au,
        mahesh@...ux.vnet.ibm.com, mhiramat@...nel.org,
        anju@...ux.vnet.ibm.com
Subject: [PATCH V2 2/4] powerpc: add helper to check if offset is within rel branch range

From: "Naveen N. Rao" <naveen.n.rao@...ux.vnet.ibm.com>

Signed-off-by: Naveen N. Rao <naveen.n.rao@...ux.vnet.ibm.com>
Signed-off-by: Anju T Sudhakar <anju@...ux.vnet.ibm.com>
---
 arch/powerpc/include/asm/code-patching.h |  1 +
 arch/powerpc/lib/code-patching.c         | 24 +++++++++++++++++++++++-
 2 files changed, 24 insertions(+), 1 deletion(-)

diff --git a/arch/powerpc/include/asm/code-patching.h b/arch/powerpc/include/asm/code-patching.h
index 2015b07..75ee4f4 100644
--- a/arch/powerpc/include/asm/code-patching.h
+++ b/arch/powerpc/include/asm/code-patching.h
@@ -22,6 +22,7 @@
 #define BRANCH_SET_LINK	0x1
 #define BRANCH_ABSOLUTE	0x2
 
+bool is_offset_in_branch_range(long offset);
 unsigned int create_branch(const unsigned int *addr,
 			   unsigned long target, int flags);
 unsigned int create_cond_branch(const unsigned int *addr,
diff --git a/arch/powerpc/lib/code-patching.c b/arch/powerpc/lib/code-patching.c
index d5edbeb..f643451 100644
--- a/arch/powerpc/lib/code-patching.c
+++ b/arch/powerpc/lib/code-patching.c
@@ -32,6 +32,28 @@ int patch_branch(unsigned int *addr, unsigned long target, int flags)
 	return patch_instruction(addr, create_branch(addr, target, flags));
 }
 
+bool is_offset_in_branch_range(long offset)
+{
+	/*
+	 * Powerpc branch instruction is :
+	 *
+	 *  0         6                 30   31
+	 *  +---------+----------------+---+---+
+	 *  | opcode  |     LI         |AA |LK |
+	 *  +---------+----------------+---+---+
+	 *  Where AA = 0 and LK = 0
+	 *
+	 * LI is a signed 24 bits integer. The real branch offset is computed
+	 * by: imm32 = SignExtend(LI:'0b00', 32);
+	 *
+	 * So the maximum forward branch should be:
+	 *   (0x007fffff << 2) = 0x01fffffc =  0x1fffffc
+	 * The maximum backward branch should be:
+	 *   (0xff800000 << 2) = 0xfe000000 = -0x2000000
+	 */
+	return (offset >= -0x2000000 && offset <= 0x1fffffc && !(offset & 0x3));
+}
+
 unsigned int create_branch(const unsigned int *addr,
 			   unsigned long target, int flags)
 {
@@ -43,7 +65,7 @@ unsigned int create_branch(const unsigned int *addr,
 		offset = offset - (unsigned long)addr;
 
 	/* Check we can represent the target in the instruction format */
-	if (offset < -0x2000000 || offset > 0x1fffffc || offset & 0x3)
+	if (!is_offset_in_branch_range(offset))
 		return 0;
 
 	/* Mask out the flags and target, so they don't step on each other. */
-- 
2.7.4