[PATCH v2 3/9] powerpc/bpf/32: Only set a stack frame when necessary

[Christophe Leroy]

Until now a stack frame was set at all time due to the need
to keep tail call counter in the stack.

But since commit fa025537f584 ("powerpc/bpf/32: Fix Oops on tail call
tests"), the tail call counter is passed via register r4. It is
therefore not necessary anymore to have a stack frame for that.

Just like PPC64, implement bpf_has_stack_frame() and only sets the frame
when needed.

The difference with PPC64 is that PPC32 doesn't have a redzone, so
the stack is required as soon as non volatile registers are used or
when tail call count is set up.

Signed-off-by: Christophe Leroy <christophe.le...@csgroup.eu>
---
 arch/powerpc/net/bpf_jit_comp32.c | 20 ++++++++++++++++++--
 1 file changed, 18 insertions(+), 2 deletions(-)

diff --git a/arch/powerpc/net/bpf_jit_comp32.c 
b/arch/powerpc/net/bpf_jit_comp32.c
index 4e6caee9c98a..7f54d37bede6 100644
--- a/arch/powerpc/net/bpf_jit_comp32.c
+++ b/arch/powerpc/net/bpf_jit_comp32.c
@@ -79,6 +79,20 @@ static int bpf_jit_stack_offsetof(struct codegen_context 
*ctx, int reg)
 #define SEEN_NVREG_FULL_MASK   0x0003ffff /* Non volatile registers r14-r31 */
 #define SEEN_NVREG_TEMP_MASK   0x00001e01 /* BPF_REG_5, BPF_REG_AX, TMP_REG */
 
+static inline bool bpf_has_stack_frame(struct codegen_context *ctx)
+{
+       /*
+        * We only need a stack frame if:
+        * - we call other functions (kernel helpers), or
+        * - we use non volatile registers, or
+        * - we use tail call counter
+        * - the bpf program uses its stack area
+        * The latter condition is deduced from the usage of BPF_REG_FP
+        */
+       return ctx->seen & (SEEN_FUNC | SEEN_TAILCALL | SEEN_NVREG_FULL_MASK) ||
+              bpf_is_seen_register(ctx, bpf_to_ppc(BPF_REG_FP));
+}
+
 void bpf_jit_realloc_regs(struct codegen_context *ctx)
 {
        unsigned int nvreg_mask;
@@ -121,7 +135,8 @@ void bpf_jit_build_prologue(u32 *image, struct 
codegen_context *ctx)
 
 #define BPF_TAILCALL_PROLOGUE_SIZE     4
 
-       EMIT(PPC_RAW_STWU(_R1, _R1, -BPF_PPC_STACKFRAME(ctx)));
+       if (bpf_has_stack_frame(ctx))
+               EMIT(PPC_RAW_STWU(_R1, _R1, -BPF_PPC_STACKFRAME(ctx)));
 
        if (ctx->seen & SEEN_TAILCALL)
                EMIT(PPC_RAW_STW(_R4, _R1, bpf_jit_stack_offsetof(ctx, 
BPF_PPC_TC)));
@@ -174,7 +189,8 @@ static void bpf_jit_emit_common_epilogue(u32 *image, struct 
codegen_context *ctx
                EMIT(PPC_RAW_LWZ(_R0, _R1, BPF_PPC_STACKFRAME(ctx) + 
PPC_LR_STKOFF));
 
        /* Tear down our stack frame */
-       EMIT(PPC_RAW_ADDI(_R1, _R1, BPF_PPC_STACKFRAME(ctx)));
+       if (bpf_has_stack_frame(ctx))
+               EMIT(PPC_RAW_ADDI(_R1, _R1, BPF_PPC_STACKFRAME(ctx)));
 
        if (ctx->seen & SEEN_FUNC)
                EMIT(PPC_RAW_MTLR(_R0));
-- 
2.39.1