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

From: Anju T Sudhakar
Date: Fri Dec 16 2016 - 11:31:31 EST


Hi Masami,


Thank you for reviewing the patch set.


On Friday 16 December 2016 05:22 PM, Masami Hiramatsu wrote:
On Wed, 14 Dec 2016 21:48:30 +0530
Anju T Sudhakar <anju@xxxxxxxxxxxxxxxxxx> wrote:

From: "Naveen N. Rao" <naveen.n.rao@xxxxxxxxxxxxxxxxxx>

The coding is OK to me. Please add a description for this patch
here, e.g. what is done by this patch, what kind of branch
instruction will be covered, and why thse checks are needed etc.


Sure. I will give a description for this patch.


Thanks and Regards,

-Anju


Thank you,

Signed-off-by: Naveen N. Rao <naveen.n.rao@xxxxxxxxxxxxxxxxxx>
Signed-off-by: Anju T Sudhakar <anju@xxxxxxxxxxxxxxxxxx>
---
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