From: Abhishek Dubey <adubey@linux.ibm.com>

The bpf_throw() function never returns, if it has
clobbered any callee-saved register, those will
remain clobbered. The prologue must take care of
saving all callee-saved registers in the frame of
exception boundary program. Later these additional
non volatile registers R14-R25 along with other
NVRs are restored back in the epilogue of exception
callback.

To achieve above objective, the frame size is
determined dynamically to accommodate additional
non volatile registers in exception boundary's frame.
For non-exception boundary program, the frame size
remains optimal. The additional instructions to
save & restore r14-r25 registers are emitted only during
exception boundary and exception callback program
respectively.

Signed-off-by: Abhishek Dubey <adubey@linux.ibm.com>
---
 arch/powerpc/net/bpf_jit_comp64.c | 87 ++++++++++++++++++++++++++++---
 1 file changed, 80 insertions(+), 7 deletions(-)

diff --git a/arch/powerpc/net/bpf_jit_comp64.c b/arch/powerpc/net/bpf_jit_comp64.c
index d7cd8ab6559c..cbaf811e1a35 100644
--- a/arch/powerpc/net/bpf_jit_comp64.c
+++ b/arch/powerpc/net/bpf_jit_comp64.c
@@ -32,21 +32,37 @@
  *
  *		[	prev sp		] <-------------
  *		[    tail_call_info	] 8		|
- *		[   nv gpr save area	] 6*8		|
+ *		[   nv gpr save area	] 6*8 + (12*8)	|
  *		[    local_tmp_var	] 24		|
  * fp (r31) -->	[   ebpf stack space	] upto 512	|
  *		[     frame header	] 32/112	|
  * sp (r1) --->	[    stack pointer	] --------------
+ *
+ * Additional (12*8) in 'nv gpr save area' only in case of
+ * exception boundary.
  */
 
 /* for bpf JIT code internal usage */
 #define BPF_PPC_STACK_LOCALS	24
+/*
+ * for additional non volatile registers(r14-r25) to be saved
+ * at exception boundary
+ */
+#define BPF_PPC_EXC_STACK_SAVE (12*8)
+
 /* stack frame excluding BPF stack, ensure this is quadword aligned */
 #define BPF_PPC_STACKFRAME	(STACK_FRAME_MIN_SIZE + \
 				 BPF_PPC_STACK_LOCALS + \
 				 BPF_PPC_STACK_SAVE   + \
 				 BPF_PPC_TAILCALL)
 
+/*
+ * same as BPF_PPC_STACKFRAME with save area for additional
+ * non volatile registers saved at exception boundary.
+ * This is quad-word aligned.
+ */
+#define BPF_PPC_EXC_STACKFRAME (BPF_PPC_STACKFRAME + BPF_PPC_EXC_STACK_SAVE)
+
 /* BPF register usage */
 #define TMP_REG_1	(MAX_BPF_JIT_REG + 0)
 #define TMP_REG_2	(MAX_BPF_JIT_REG + 1)
@@ -103,9 +119,12 @@ static inline bool bpf_has_stack_frame(struct codegen_context *ctx)
  *		[	  ...       	] 		|
  * sp (r1) --->	[    stack pointer	] --------------
  *		[    tail_call_info	] 8
- *		[   nv gpr save area	] 6*8
+ *		[   nv gpr save area	] 6*8 + (12*8)
  *		[    local_tmp_var	] 24
  *		[   unused red zone	] 224
+ *
+ * Additional (12*8) in 'nv gpr save area' only in case of
+ * exception boundary.
  */
 static int bpf_jit_stack_local(struct codegen_context *ctx)
 {
@@ -114,7 +133,12 @@ static int bpf_jit_stack_local(struct codegen_context *ctx)
 		return STACK_FRAME_MIN_SIZE + ctx->stack_size;
 	} else {
 		/* Stack layout with redzone */
-		return -(BPF_PPC_TAILCALL + BPF_PPC_STACK_SAVE + BPF_PPC_STACK_LOCALS);
+		return -(BPF_PPC_TAILCALL
+			+BPF_PPC_STACK_SAVE
+			+(ctx->exception_boundary || ctx->exception_cb ?
+						BPF_PPC_EXC_STACK_SAVE : 0)
+			+BPF_PPC_STACK_LOCALS
+			);
 	}
 }
 
@@ -125,9 +149,19 @@ int bpf_jit_stack_tailcallinfo_offset(struct codegen_context *ctx)
 
 static int bpf_jit_stack_offsetof(struct codegen_context *ctx, int reg)
 {
-	if (reg >= BPF_PPC_NVR_MIN && reg < 32)
+	int min_valid_nvreg = BPF_PPC_NVR_MIN;
+	/* Default frame size for all cases except exception boundary */
+	int frame_nvr_size = BPF_PPC_STACKFRAME;
+
+	/* Consider all nv regs for handling exceptions */
+	if (ctx->exception_boundary || ctx->exception_cb) {
+		min_valid_nvreg = _R14;
+		frame_nvr_size = BPF_PPC_EXC_STACKFRAME;
+	}
+
+	if (reg >= min_valid_nvreg && reg < 32)
 		return (bpf_has_stack_frame(ctx) ?
-			(BPF_PPC_STACKFRAME + ctx->stack_size) : 0)
+			(frame_nvr_size + ctx->stack_size) : 0)
 				- (8 * (32 - reg)) - BPF_PPC_TAILCALL;
 
 	pr_err("BPF JIT is asking about unknown registers");
@@ -138,6 +172,17 @@ void bpf_jit_realloc_regs(struct codegen_context *ctx)
 {
 }
 
+/*
+ * For exception boundary & exception_cb progs:
+ *     return increased size to accommodate additional NVRs.
+ */
+static int bpf_jit_stack_size(struct codegen_context *ctx)
+{
+	return ctx->exception_boundary || ctx->exception_cb ?
+					BPF_PPC_EXC_STACKFRAME :
+					BPF_PPC_STACKFRAME;
+}
+
 void bpf_jit_build_prologue(u32 *image, struct codegen_context *ctx)
 {
 	int i;
@@ -198,7 +243,19 @@ void bpf_jit_build_prologue(u32 *image, struct codegen_context *ctx)
 			EMIT(PPC_RAW_STD(_R0, _R1, PPC_LR_STKOFF));
 		}
 
-		EMIT(PPC_RAW_STDU(_R1, _R1, -(BPF_PPC_STACKFRAME + ctx->stack_size)));
+		EMIT(PPC_RAW_STDU(_R1, _R1,
+				-(bpf_jit_stack_size(ctx) + ctx->stack_size)));
+	}
+
+	/*
+	 * Program acting as exception boundary pushes R14..R25 in addition to
+	 * BPF callee-saved non volatile registers. Exception callback uses
+	 * the boundary program's stack frame, recover additionally saved
+	 * registers in epilogue of exception callback.
+	 */
+	if (ctx->exception_boundary) {
+		for (i = _R14; i <= _R25; i++)
+			EMIT(PPC_RAW_STD(i, _R1, bpf_jit_stack_offsetof(ctx, i)));
 	}
 
 	if (!ctx->exception_cb) {
@@ -245,9 +302,25 @@ static void bpf_jit_emit_common_epilogue(u32 *image, struct codegen_context *ctx
 		EMIT(PPC_RAW_LD(bpf_to_ppc(ARENA_VM_START), _R1,
 				bpf_jit_stack_offsetof(ctx, bpf_to_ppc(ARENA_VM_START))));
 
+	if (ctx->exception_cb) {
+		/*
+		 * Recover additionally saved non volatile registers from stack
+		 * frame of exception boundary program.
+		 */
+		for (i = _R14; i <= _R25; i++)
+			EMIT(PPC_RAW_LD(i, _R1, bpf_jit_stack_offsetof(ctx, i)));
+
+		/*
+		 * LR value from boundary-frame is received as second parameter
+		 * in exception callback.
+		 */
+		EMIT(PPC_RAW_MTLR(_R4));
+	}
+
 	/* Tear down our stack frame */
 	if (bpf_has_stack_frame(ctx)) {
-		EMIT(PPC_RAW_ADDI(_R1, _R1, BPF_PPC_STACKFRAME + ctx->stack_size));
+		EMIT(PPC_RAW_ADDI(_R1, _R1, bpf_jit_stack_size(ctx) + ctx->stack_size));
+
 		if (ctx->seen & SEEN_FUNC || ctx->exception_cb) {
 			EMIT(PPC_RAW_LD(_R0, _R1, PPC_LR_STKOFF));
 			EMIT(PPC_RAW_MTLR(_R0));
-- 
2.48.1

> diff --git a/arch/powerpc/net/bpf_jit_comp64.c b/arch/powerpc/net/bpf_jit_comp64.c
> index d7cd8ab6559c..cbaf811e1a35 100644
> --- a/arch/powerpc/net/bpf_jit_comp64.c
> +++ b/arch/powerpc/net/bpf_jit_comp64.c

[ ... ]

> @@ -245,9 +302,25 @@ static void bpf_jit_emit_common_epilogue(u32 *image, struct codegen_context *ctx
>  		EMIT(PPC_RAW_LD(bpf_to_ppc(ARENA_VM_START), _R1,
>  				bpf_jit_stack_offsetof(ctx, bpf_to_ppc(ARENA_VM_START))));
>
> +	if (ctx->exception_cb) {
> +		/*
> +		 * Recover additionally saved non volatile registers from stack
> +		 * frame of exception boundary program.
> +		 */
> +		for (i = _R14; i <= _R25; i++)
> +			EMIT(PPC_RAW_LD(i, _R1, bpf_jit_stack_offsetof(ctx, i)));
> +
> +		/*
> +		 * LR value from boundary-frame is received as second parameter
> +		 * in exception callback.
> +		 */
> +		EMIT(PPC_RAW_MTLR(_R4));
                ^^^^^^^^^^^^^^^^^^^

This sets LR from R4 (the boundary's LR passed as second parameter).

> +	}
> +
>  	/* Tear down our stack frame */
>  	if (bpf_has_stack_frame(ctx)) {
> -		EMIT(PPC_RAW_ADDI(_R1, _R1, BPF_PPC_STACKFRAME + ctx->stack_size));
> +		EMIT(PPC_RAW_ADDI(_R1, _R1, bpf_jit_stack_size(ctx) + ctx->stack_size));
> +
>  		if (ctx->seen & SEEN_FUNC || ctx->exception_cb) {
                                          ^^^^^^^^^^^^^^^^^^^^^
>  			EMIT(PPC_RAW_LD(_R0, _R1, PPC_LR_STKOFF));
>  			EMIT(PPC_RAW_MTLR(_R0));
                        ^^^^^^^^^^^^^^^^^^^

For exception_cb programs, bpf_has_stack_frame() returns true (the function
explicitly checks ctx->exception_cb). The condition on the inner if statement
also evaluates true because of "|| ctx->exception_cb".

This means for exception_cb programs, LR is first set correctly from R4,
but then immediately overwritten by loading from the stack and issuing a
second MTLR.

The comment above the EMIT(PPC_RAW_MTLR(_R4)) states that LR is received as
the second parameter, suggesting R4 is the authoritative source. If
exception_cb and exception_boundary have different stack_depth values, the
stack-loaded LR value after the ADDI may point to an incorrect location.

Should the inner condition exclude exception_cb, perhaps something like:

    if (ctx->seen & SEEN_FUNC && !ctx->exception_cb) {

Or alternatively, should the EMIT(PPC_RAW_MTLR(_R4)) be moved after the
stack teardown code to ensure it takes precedence?


---
AI reviewed your patch. Please fix the bug or email reply why it's not a bug.
See: https://github.com/kernel-patches/vmtest/blob/master/ci/claude/README.md

CI run summary: https://github.com/kernel-patches/bpf/actions/runs/21257691486