[RFC PATCH v3 08/11] powerpc/ftrace: Add support for DYNAMIC_FTRACE_WITH_DIRECT_CALLS

Naveen N Rao
2024-06-20T18:57:56Z

Add support for DYNAMIC_FTRACE_WITH_DIRECT_CALLS similar to the arm64
implementation.

ftrace direct calls allow custom trampolines to be called into directly
from function ftrace call sites, bypassing the ftrace trampoline
completely. This functionality is currently utilized by BPF trampolines
to hook into kernel function entries.

Since we have limited relative branch range, we support ftrace direct
calls through support for DYNAMIC_FTRACE_WITH_CALL_OPS. In this
approach, ftrace trampoline is not entirely bypassed. Rather, it is
re-purposed into a stub that reads direct_call field from the associated
ftrace_ops structure and branches into that, if it is not NULL. For
this, it is sufficient if we can ensure that the ftrace trampoline is
reachable from all traceable functions.

When multiple ftrace_ops are associated with a call site, we utilize a
call back to set pt_regs->orig_gpr3 that can then be tested on the
return path from the ftrace trampoline to branch into the direct caller.

Signed-off-by: Naveen N Rao <naveen@kernel.org>
---
arch/powerpc/Kconfig | 1 +
arch/powerpc/include/asm/ftrace.h | 15 ++++
arch/powerpc/kernel/asm-offsets.c | 3 +
arch/powerpc/kernel/trace/ftrace.c | 9 +++
arch/powerpc/kernel/trace/ftrace_entry.S | 99 ++++++++++++++++++------
5 files changed, 105 insertions(+), 22 deletions(-)

diff --git a/arch/powerpc/Kconfig b/arch/powerpc/Kconfig
index fde64ad19de5..96ae653bdcde 100644
--- a/arch/powerpc/Kconfig
+++ b/arch/powerpc/Kconfig
@@ -236,6 +236,7 @@ config PPC
select HAVE_DYNAMIC_FTRACE
select HAVE_DYNAMIC_FTRACE_WITH_ARGS if ARCH_USING_PATCHABLE_FUNCTION_ENTRY || MPROFILE_KERNEL || PPC32
select HAVE_DYNAMIC_FTRACE_WITH_CALL_OPS if FTRACE_PFE_OUT_OF_LINE || (PPC32 && ARCH_USING_PATCHABLE_FUNCTION_ENTRY)
+ select HAVE_DYNAMIC_FTRACE_WITH_DIRECT_CALLS if HAVE_DYNAMIC_FTRACE_WITH_CALL_OPS
select HAVE_DYNAMIC_FTRACE_WITH_REGS if ARCH_USING_PATCHABLE_FUNCTION_ENTRY || MPROFILE_KERNEL || PPC32
select HAVE_EBPF_JIT
select HAVE_EFFICIENT_UNALIGNED_ACCESS
diff --git a/arch/powerpc/include/asm/ftrace.h b/arch/powerpc/include/asm/ftrace.h
index 938cecf72eb1..fc0f25b10e86 100644
--- a/arch/powerpc/include/asm/ftrace.h
+++ b/arch/powerpc/include/asm/ftrace.h
@@ -147,6 +147,21 @@ extern unsigned long ftrace_pfe_stub_text_count, ftrace_pfe_stub_inittext_count;
#endif
void ftrace_free_init_tramp(void);
unsigned long ftrace_call_adjust(unsigned long addr);
+
+#ifdef CONFIG_DYNAMIC_FTRACE_WITH_DIRECT_CALLS
+/*
+ * When an ftrace registered caller is tracing a function that is also set by a
+ * register_ftrace_direct() call, it needs to be differentiated in the
+ * ftrace_caller trampoline so that the direct call can be invoked after the
+ * other ftrace ops. To do this, place the direct caller in the orig_gpr3 field
+ * of pt_regs. This tells ftrace_caller that there's a direct caller.
+ */
+static inline void arch_ftrace_set_direct_caller(struct ftrace_regs *fregs, unsigned long addr)
+{
+ struct pt_regs *regs = &fregs->regs;
+ regs->orig_gpr3 = addr;
+}
+#endif /* CONFIG_DYNAMIC_FTRACE_WITH_DIRECT_CALLS */
#else
static inline void ftrace_free_init_tramp(void) { }
static inline unsigned long ftrace_call_adjust(unsigned long addr) { return addr; }
diff --git a/arch/powerpc/kernel/asm-offsets.c b/arch/powerpc/kernel/asm-offsets.c
index a11ea5f4d86a..0b955dddeb28 100644
--- a/arch/powerpc/kernel/asm-offsets.c
+++ b/arch/powerpc/kernel/asm-offsets.c
@@ -680,6 +680,9 @@ int main(void)

#ifdef CONFIG_DYNAMIC_FTRACE_WITH_CALL_OPS
OFFSET(FTRACE_OPS_FUNC, ftrace_ops, func);
+#ifdef CONFIG_DYNAMIC_FTRACE_WITH_DIRECT_CALLS
+ OFFSET(FTRACE_OPS_DIRECT_CALL, ftrace_ops, direct_call);
+#endif
#endif

return 0;
diff --git a/arch/powerpc/kernel/trace/ftrace.c b/arch/powerpc/kernel/trace/ftrace.c
index 028548312c23..799612ee270f 100644
--- a/arch/powerpc/kernel/trace/ftrace.c
+++ b/arch/powerpc/kernel/trace/ftrace.c
@@ -153,6 +153,15 @@ static int ftrace_get_call_inst(struct dyn_ftrace *rec, unsigned long addr, ppc_
if (IS_ENABLED(CONFIG_FTRACE_PFE_OUT_OF_LINE))
ip = ftrace_get_pfe_stub(rec) + MCOUNT_INSN_SIZE; /* second instruction in stub */

+ if (!is_offset_in_branch_range(addr - ip) && addr != FTRACE_ADDR && addr != FTRACE_REGS_ADDR) {
+ /* This can only happen with ftrace direct */
+ if (!IS_ENABLED(CONFIG_DYNAMIC_FTRACE_WITH_DIRECT_CALLS)) {
+ pr_err("0x%lx (0x%lx): Unexpected target address 0x%lx\n", ip, rec->ip, addr);
+ return -EINVAL;
+ }
+ addr = FTRACE_ADDR;
+ }
+
if (is_offset_in_branch_range(addr - ip))
/* Within range */
stub = addr;
diff --git a/arch/powerpc/kernel/trace/ftrace_entry.S b/arch/powerpc/kernel/trace/ftrace_entry.S
index a76aedd970a6..d5a63e60aafa 100644
--- a/arch/powerpc/kernel/trace/ftrace_entry.S
+++ b/arch/powerpc/kernel/trace/ftrace_entry.S
@@ -33,14 +33,38 @@
* and then arrange for the ftrace function to be called.
*/
.macro ftrace_regs_entry allregs
- /* Save the original return address in A's stack frame */
- PPC_STL r0, LRSAVE(r1)
/* Create a minimal stack frame for representing B */
PPC_STLU r1, -STACK_FRAME_MIN_SIZE(r1)

/* Create our stack frame + pt_regs */
PPC_STLU r1,-SWITCH_FRAME_SIZE(r1)

+ .if \allregs == 1
+ SAVE_GPRS(11, 12, r1)
+ .endif
+
+ /* Get the _mcount() call site out of LR */
+ mflr r11
+
+#ifdef CONFIG_DYNAMIC_FTRACE_WITH_DIRECT_CALLS
+ /* Load the ftrace_op */
+ PPC_LL r12, -(MCOUNT_INSN_SIZE*2 + SZL)(r11)
+
+ /* Load direct_call from the ftrace_op */
+ PPC_LL r12, FTRACE_OPS_DIRECT_CALL(r12)
+ PPC_LCMPI r12, 0
+ .if \allregs == 1
+ bne .Lftrace_direct_call_regs
+ .else
+ bne .Lftrace_direct_call
+ .endif
+#endif
+
+ /* Save the previous LR in pt_regs->link */
+ PPC_STL r0, _LINK(r1)
+ /* Also save it in A's stack frame */
+ PPC_STL r0, SWITCH_FRAME_SIZE+STACK_FRAME_MIN_SIZE+LRSAVE(r1)
+
/* Save all gprs to pt_regs */
SAVE_GPR(0, r1)
SAVE_GPRS(3, 10, r1)
@@ -54,7 +78,7 @@

.if \allregs == 1
SAVE_GPR(2, r1)
- SAVE_GPRS(11, 31, r1)
+ SAVE_GPRS(13, 31, r1)
.else
#ifdef CONFIG_LIVEPATCH_64
SAVE_GPR(14, r1)
@@ -67,20 +91,15 @@

.if \allregs == 1
/* Load special regs for save below */
+ mfcr r7
mfmsr r8
mfctr r9
mfxer r10
- mfcr r11
.else
/* Clear MSR to flag as ftrace_caller versus frace_regs_caller */
li r8, 0
.endif

- /* Get the _mcount() call site out of LR */
- mflr r7
- /* Save the read LR in pt_regs->link */
- PPC_STL r0, _LINK(r1)
-
#ifdef CONFIG_PPC64
/* Save callee's TOC in the ABI compliant location */
std r2, STK_GOT(r1)
@@ -88,8 +107,8 @@
#endif

#ifdef CONFIG_DYNAMIC_FTRACE_WITH_CALL_OPS
- /* r7 points to the instruction following the call to ftrace */
- PPC_LL r5, -(MCOUNT_INSN_SIZE*2 + SZL)(r7)
+ /* r11 points to the instruction following the call to ftrace */
+ PPC_LL r5, -(MCOUNT_INSN_SIZE*2 + SZL)(r11)
PPC_LL r12, FTRACE_OPS_FUNC(r5)
mtctr r12
#else /* !CONFIG_DYNAMIC_FTRACE_WITH_CALL_OPS */
@@ -105,40 +124,47 @@
/* Save special regs */
PPC_STL r8, _MSR(r1)
.if \allregs == 1
+ PPC_STL r7, _CCR(r1)
PPC_STL r9, _CTR(r1)
PPC_STL r10, _XER(r1)
- PPC_STL r11, _CCR(r1)
.endif

+#ifdef CONFIG_DYNAMIC_FTRACE_WITH_DIRECT_CALLS
+ /* Clear orig_gpr3 to later detect ftrace_direct call */
+ li r7, 0
+ PPC_STL r7, ORIG_GPR3(r1)
+#endif
+
#ifdef CONFIG_FTRACE_PFE_OUT_OF_LINE
/* Save our real return address locally for return */
- PPC_STL r7, STACK_INT_FRAME_MARKER(r1)
+ PPC_STL r11, STACK_INT_FRAME_MARKER(r1)
/*
- * We want the ftrace location in the function, but our lr (in r7)
+ * We want the ftrace location in the function, but our lr (in r11)
* points at the 'mtlr r0' instruction in the out of line stub. To
* recover the ftrace location, we read the branch instruction in the
* stub, and adjust our lr by the branch offset.
*/
- lwz r8, MCOUNT_INSN_SIZE(r7)
+ lwz r8, MCOUNT_INSN_SIZE(r11)
slwi r8, r8, 6
srawi r8, r8, 6
- add r3, r7, r8
+ add r3, r11, r8
/*
* Override our nip to point past the branch in the original function.
* This allows reliable stack trace and the ftrace stack tracer to work as-is.
*/
- add r7, r3, MCOUNT_INSN_SIZE
+ add r11, r3, MCOUNT_INSN_SIZE
#else
/* Calculate ip from nip-4 into r3 for call below */
- subi r3, r7, MCOUNT_INSN_SIZE
+ subi r3, r11, MCOUNT_INSN_SIZE
#endif

/* Save NIP as pt_regs->nip */
- PPC_STL r7, _NIP(r1)
+ PPC_STL r11, _NIP(r1)
/* Also save it in B's stackframe header for proper unwind */
- PPC_STL r7, LRSAVE+SWITCH_FRAME_SIZE(r1)
+ PPC_STL r11, LRSAVE+SWITCH_FRAME_SIZE(r1)
+
#ifdef CONFIG_LIVEPATCH_64
- mr r14, r7 /* remember old NIP */
+ mr r14, r11 /* remember old NIP */
#endif

/* Put the original return address in r4 as parent_ip */
@@ -149,11 +175,21 @@
.endm

.macro ftrace_regs_exit allregs
+#ifdef CONFIG_DYNAMIC_FTRACE_WITH_DIRECT_CALLS
+ /* Check orig_gpr3 to detect ftrace_direct call */
+ PPC_LL r7, ORIG_GPR3(r1)
+ PPC_LCMPI cr1, r7, 0
+ mtctr r7
+#endif
+
#ifndef CONFIG_FTRACE_PFE_OUT_OF_LINE
/* Load ctr with the possibly modified NIP */
PPC_LL r3, _NIP(r1)
+#ifdef CONFIG_DYNAMIC_FTRACE_WITH_DIRECT_CALLS
+ bne cr1,2f
+#endif
mtctr r3
-
+2:
#ifdef CONFIG_LIVEPATCH_64
cmpd r14, r3 /* has NIP been altered? */
#endif
@@ -198,7 +234,11 @@
/* Based on the cmpd above, if the NIP was altered handle livepatch */
bne- livepatch_handler
#endif
+
/* jump after _mcount site */
+#ifdef CONFIG_DYNAMIC_FTRACE_WITH_DIRECT_CALLS
+ bnectr cr1
+#endif
#ifdef CONFIG_FTRACE_PFE_OUT_OF_LINE
blr
#else
@@ -251,6 +291,21 @@ ftrace_no_trace:
#endif
#endif

+#ifdef CONFIG_DYNAMIC_FTRACE_WITH_DIRECT_CALLS
+.Lftrace_direct_call_regs:
+ mtctr r12
+ REST_GPRS(11, 12, r1)
+ addi r1, r1, SWITCH_FRAME_SIZE+STACK_FRAME_MIN_SIZE
+ bctr
+.Lftrace_direct_call:
+ mtctr r12
+ addi r1, r1, SWITCH_FRAME_SIZE+STACK_FRAME_MIN_SIZE
+ bctr
+SYM_FUNC_START(ftrace_stub_direct_tramp)
+ blr
+SYM_FUNC_END(ftrace_stub_direct_tramp)
+#endif
+
#ifdef CONFIG_LIVEPATCH_64
/*
* This function runs in the mcount context, between two functions. As
--
2.45.2