[PATCH bpf v3] selftests/bpf: Fix "missing ENDBR" BUG for destructor kfunc

[Chen Hu <hu1.chen@xxxxxxxxx>]

With CONFIG_X86_KERNEL_IBT enabled, the test_verifier triggers the
following BUG:

  traps: Missing ENDBR: bpf_kfunc_call_test_release+0x0/0x30
  ------------[ cut here ]------------
  kernel BUG at arch/x86/kernel/traps.c:254!
  invalid opcode: 0000 [#1] PREEMPT SMP
  <TASK>
   asm_exc_control_protection+0x26/0x50
  RIP: 0010:bpf_kfunc_call_test_release+0x0/0x30
  Code: 00 48 c7 c7 18 f2 e1 b4 e8 0d ca 8c ff 48 c7 c0 00 f2 e1 b4 c3
	0f 1f 44 00 00 66 0f 1f 00 0f 1f 44 00 00 0f 0b 31 c0 c3 66 90
       <66> 0f 1f 00 0f 1f 44 00 00 48 85 ff 74 13 4c 8d 47 18 b8 ff ff ff
   bpf_map_free_kptrs+0x2e/0x70
   array_map_free+0x57/0x140
   process_one_work+0x194/0x3a0
   worker_thread+0x54/0x3a0
   ? rescuer_thread+0x390/0x390
   kthread+0xe9/0x110
   ? kthread_complete_and_exit+0x20/0x20

It turns out that ENDBR in bpf_kfunc_call_test_release() is converted to
NOP by apply_ibt_endbr().

The only text references to this function from kernel side are:

  $ grep -r bpf_kfunc_call_test_release
  net/bpf/test_run.c:noinline void bpf_kfunc_call_test_release(...)
  net/bpf/test_run.c:BTF_ID_FLAGS(func, bpf_kfunc_call_test_release, ...)
  net/bpf/test_run.c:BTF_ID(func, bpf_kfunc_call_test_release)

but it may be called from bpf program as kfunc. (no other caller from
kernel)

This fix creates dummy references to destructor kfuncs so ENDBR stay
there.

Also modify macro XXX_NOSEAL slightly:
- ASM_IBT_NOSEAL now stands for pure asm
- IBT_NOSEAL can be used directly in C

Signed-off-by: Chen Hu <hu1.chen@xxxxxxxxx>
Tested-by: Pengfei Xu <pengfei.xu@xxxxxxxxx>
---
v3:
- Macro go to IBT related header as suggested by Jiri Olsa
- Describe reference to the func clearly in commit message as suggested
  by Peter Zijlstra and Jiri Olsa
 
v2: https://lore.kernel.org/all/20221122073244.21279-1-hu1.chen@xxxxxxxxx/

v1: https://lore.kernel.org/all/20221121085113.611504-1-hu1.chen@xxxxxxxxx/

 arch/x86/include/asm/ibt.h | 6 +++++-
 arch/x86/kvm/emulate.c     | 2 +-
 net/bpf/test_run.c         | 5 +++++
 3 files changed, 11 insertions(+), 2 deletions(-)

diff --git a/arch/x86/include/asm/ibt.h b/arch/x86/include/asm/ibt.h
index 9b08082a5d9f..be86dc31661c 100644
--- a/arch/x86/include/asm/ibt.h
+++ b/arch/x86/include/asm/ibt.h
@@ -36,11 +36,14 @@
  * the function as needing to be "sealed" (i.e. ENDBR converted to NOP by
  * apply_ibt_endbr()).
  */
-#define IBT_NOSEAL(fname)				\
+#define ASM_IBT_NOSEAL(fname)				\
 	".pushsection .discard.ibt_endbr_noseal\n\t"	\
 	_ASM_PTR fname "\n\t"				\
 	".popsection\n\t"
 
+#define IBT_NOSEAL(name)				\
+	asm(ASM_IBT_NOSEAL(#name))
+
 static inline __attribute_const__ u32 gen_endbr(void)
 {
 	u32 endbr;
@@ -94,6 +97,7 @@ extern __noendbr void ibt_restore(u64 save);
 #ifndef __ASSEMBLY__
 
 #define ASM_ENDBR
+#define ASM_IBT_NOSEAL(name)
 #define IBT_NOSEAL(name)
 
 #define __noendbr
diff --git a/arch/x86/kvm/emulate.c b/arch/x86/kvm/emulate.c
index 4a43261d25a2..d870c8bb5831 100644
--- a/arch/x86/kvm/emulate.c
+++ b/arch/x86/kvm/emulate.c
@@ -327,7 +327,7 @@ static int fastop(struct x86_emulate_ctxt *ctxt, fastop_t fop);
 	".type " name ", @function \n\t" \
 	name ":\n\t" \
 	ASM_ENDBR \
-	IBT_NOSEAL(name)
+	ASM_IBT_NOSEAL(name)
 
 #define FOP_FUNC(name) \
 	__FOP_FUNC(#name)
diff --git a/net/bpf/test_run.c b/net/bpf/test_run.c
index fcb3e6c5e03c..9e9c8e8d50d7 100644
--- a/net/bpf/test_run.c
+++ b/net/bpf/test_run.c
@@ -601,6 +601,11 @@ noinline void bpf_kfunc_call_memb_release(struct prog_test_member *p)
 {
 }
 
+#ifdef CONFIG_X86_KERNEL_IBT
+IBT_NOSEAL(bpf_kfunc_call_test_release);
+IBT_NOSEAL(bpf_kfunc_call_memb_release);
+#endif
+
 noinline void bpf_kfunc_call_memb1_release(struct prog_test_member1 *p)
 {
 	WARN_ON_ONCE(1);
-- 
2.34.1