From: Jiri Olsa <jolsa@redhat.com>
Date: Wed, 8 Dec 2021 20:32:43 +0100
Subject: bpf, x64: Replace some stack_size usage with offset variables
Patch-mainline: v5.17-rc1
Git-commit: 5edf6a1983b90371da888ca86493937ec1c8a2b5
References: jsc#PED-1368

As suggested by Andrii, adding variables for registers and ip
address offsets, which makes the code more clear, rather than
abusing single stack_size variable for everything.

Also describing the stack layout in the comment.

There is no function change.

Suggested-by: Andrii Nakryiko <andrii@kernel.org>
Signed-off-by: Jiri Olsa <jolsa@kernel.org>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Acked-by: John Fastabend <john.fastabend@gmail.com>
Acked-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/20211208193245.172141-4-jolsa@kernel.org
Acked-by: Shung-Hsi Yu <shung-hsi.yu@suse.com>
---
 arch/x86/net/bpf_jit_comp.c |   42 ++++++++++++++++++++++++++++--------------
 1 file changed, 28 insertions(+), 14 deletions(-)

--- a/arch/x86/net/bpf_jit_comp.c
+++ b/arch/x86/net/bpf_jit_comp.c
@@ -1933,7 +1933,7 @@ int arch_prepare_bpf_trampoline(struct b
 				void *orig_call)
 {
 	int ret, i, nr_args = m->nr_args;
-	int stack_size = nr_args * 8;
+	int regs_off, ip_off, stack_size = nr_args * 8;
 	struct bpf_tramp_progs *fentry = &tprogs[BPF_TRAMP_FENTRY];
 	struct bpf_tramp_progs *fexit = &tprogs[BPF_TRAMP_FEXIT];
 	struct bpf_tramp_progs *fmod_ret = &tprogs[BPF_TRAMP_MODIFY_RETURN];
@@ -1948,14 +1948,33 @@ int arch_prepare_bpf_trampoline(struct b
 	if (!is_valid_bpf_tramp_flags(flags))
 		return -EINVAL;
 
+	/* Generated trampoline stack layout:
+	 *
+	 * RBP + 8         [ return address  ]
+	 * RBP + 0         [ RBP             ]
+	 *
+	 * RBP - 8         [ return value    ]  BPF_TRAMP_F_CALL_ORIG or
+	 *                                      BPF_TRAMP_F_RET_FENTRY_RET flags
+	 *
+	 *                 [ reg_argN        ]  always
+	 *                 [ ...             ]
+	 * RBP - regs_off  [ reg_arg1        ]  program's ctx pointer
+	 *
+	 * RBP - ip_off    [ traced function ]  BPF_TRAMP_F_IP_ARG flag
+	 */
+
 	/* room for return value of orig_call or fentry prog */
 	save_ret = flags & (BPF_TRAMP_F_CALL_ORIG | BPF_TRAMP_F_RET_FENTRY_RET);
 	if (save_ret)
 		stack_size += 8;
 
+	regs_off = stack_size;
+
 	if (flags & BPF_TRAMP_F_IP_ARG)
 		stack_size += 8; /* room for IP address argument */
 
+	ip_off = stack_size;
+
 	if (flags & BPF_TRAMP_F_SKIP_FRAME)
 		/* skip patched call instruction and point orig_call to actual
 		 * body of the kernel function.
@@ -1973,19 +1992,14 @@ int arch_prepare_bpf_trampoline(struct b
 		/* Store IP address of the traced function:
 		 * mov rax, QWORD PTR [rbp + 8]
 		 * sub rax, X86_PATCH_SIZE
-		 * mov QWORD PTR [rbp - stack_size], rax
+		 * mov QWORD PTR [rbp - ip_off], rax
 		 */
 		emit_ldx(&prog, BPF_DW, BPF_REG_0, BPF_REG_FP, 8);
 		EMIT4(0x48, 0x83, 0xe8, X86_PATCH_SIZE);
-		emit_stx(&prog, BPF_DW, BPF_REG_FP, BPF_REG_0, -stack_size);
-
-		/* Continue with stack_size for regs storage, stack will
-		 * be correctly restored with 'leave' instruction.
-		 */
-		stack_size -= 8;
+		emit_stx(&prog, BPF_DW, BPF_REG_FP, BPF_REG_0, -ip_off);
 	}
 
-	save_regs(m, &prog, nr_args, stack_size);
+	save_regs(m, &prog, nr_args, regs_off);
 
 	if (flags & BPF_TRAMP_F_CALL_ORIG) {
 		/* arg1: mov rdi, im */
@@ -1997,7 +2011,7 @@ int arch_prepare_bpf_trampoline(struct b
 	}
 
 	if (fentry->nr_progs)
-		if (invoke_bpf(m, &prog, fentry, stack_size,
+		if (invoke_bpf(m, &prog, fentry, regs_off,
 			       flags & BPF_TRAMP_F_RET_FENTRY_RET))
 			return -EINVAL;
 
@@ -2007,7 +2021,7 @@ int arch_prepare_bpf_trampoline(struct b
 		if (!branches)
 			return -ENOMEM;
 
-		if (invoke_bpf_mod_ret(m, &prog, fmod_ret, stack_size,
+		if (invoke_bpf_mod_ret(m, &prog, fmod_ret, regs_off,
 				       branches)) {
 			ret = -EINVAL;
 			goto cleanup;
@@ -2015,7 +2029,7 @@ int arch_prepare_bpf_trampoline(struct b
 	}
 
 	if (flags & BPF_TRAMP_F_CALL_ORIG) {
-		restore_regs(m, &prog, nr_args, stack_size);
+		restore_regs(m, &prog, nr_args, regs_off);
 
 		/* call original function */
 		if (emit_call(&prog, orig_call, prog)) {
@@ -2045,13 +2059,13 @@ int arch_prepare_bpf_trampoline(struct b
 	}
 
 	if (fexit->nr_progs)
-		if (invoke_bpf(m, &prog, fexit, stack_size, false)) {
+		if (invoke_bpf(m, &prog, fexit, regs_off, false)) {
 			ret = -EINVAL;
 			goto cleanup;
 		}
 
 	if (flags & BPF_TRAMP_F_RESTORE_REGS)
-		restore_regs(m, &prog, nr_args, stack_size);
+		restore_regs(m, &prog, nr_args, regs_off);
 
 	/* This needs to be done regardless. If there were fmod_ret programs,
 	 * the return value is only updated on the stack and still needs to be