[PATCH v3] powerpc/lockdep: fix a false positive warning

The commit 108c14858b9e ("locking/lockdep: Add support for dynamic
keys") introduced a boot warning on powerpc below, because since the
commit 2d4f567103ff ("KVM: PPC: Introduce kvm_tmp framework") adds
kvm_tmp[] into the .bss section and then free the rest of unused spaces
back to the page allocator.

kernel_init
  kvm_guest_init
    kvm_free_tmp
      free_reserved_area
        free_unref_page
          free_unref_page_prepare

Later, alloc_workqueue() happens to allocate some pages from there and
trigger the warning at,

if (WARN_ON_ONCE(static_obj(key)))

Fix it by adding a generic helper arch_is_bss_hole() to skip those areas
in static_obj(). Since kvm_free_tmp() is only done early during the
boot, just go lockless to make the implementation simple for now.

WARNING: CPU: 0 PID: 13 at kernel/locking/lockdep.c:1120
Workqueue: events work_for_cpu_fn
Call Trace:
  lockdep_register_key+0x68/0x200
  wq_init_lockdep+0x40/0xc0
  trunc_msg+0x385f9/0x4c30f (unreliable)
  wq_init_lockdep+0x40/0xc0
  alloc_workqueue+0x1e0/0x620
  scsi_host_alloc+0x3d8/0x490
  ata_scsi_add_hosts+0xd0/0x220 [libata]
  ata_host_register+0x178/0x400 [libata]
  ata_host_activate+0x17c/0x210 [libata]
  ahci_host_activate+0x84/0x250 [libahci]
  ahci_init_one+0xc74/0xdc0 [ahci]
  local_pci_probe+0x78/0x100
  work_for_cpu_fn+0x40/0x70
  process_one_work+0x388/0x750
  process_scheduled_works+0x50/0x90
  worker_thread+0x3d0/0x570
  kthread+0x1b8/0x1e0
  ret_from_kernel_thread+0x5c/0x7c

Fixes: 108c14858b9e ("locking/lockdep: Add support for dynamic keys")
Signed-off-by: Qian Cai <redacted>
---

v3: Change arch_is_bss_hole() to return a "bool".
    Rearrange variables in kvm.c a bit.
v2: No need to actually define arch_is_bss_hole() powerpc64 only.

 arch/powerpc/include/asm/sections.h | 11 +++++++++++
 arch/powerpc/kernel/kvm.c           |  8 +++++++-
 include/asm-generic/sections.h      |  7 +++++++
 kernel/locking/lockdep.c            |  3 +++
 4 files changed, 28 insertions(+), 1 deletion(-)

diff --git a/arch/powerpc/include/asm/sections.h b/arch/powerpc/include/asm/sections.h
index 4a1664a8658d..6e9e39ebbb27 100644
--- a/arch/powerpc/include/asm/sections.h
+++ b/arch/powerpc/include/asm/sections.h

@@ -5,8 +5,19 @@
 
 #include <linux/elf.h>
 #include <linux/uaccess.h>
+
+#define arch_is_bss_hole arch_is_bss_hole
+
 #include <asm-generic/sections.h>
 
+extern void *bss_hole_start, *bss_hole_end;
+
+static inline bool arch_is_bss_hole(unsigned long addr)
+{
+	return addr >= (unsigned long)bss_hole_start &&
+	       addr < (unsigned long)bss_hole_end;
+}
+
 extern char __head_end[];
 
 #ifdef __powerpc64__

diff --git a/arch/powerpc/kernel/kvm.c b/arch/powerpc/kernel/kvm.c
index b7b3a5e4e224..e3c3b076ff07 100644
--- a/arch/powerpc/kernel/kvm.c
+++ b/arch/powerpc/kernel/kvm.c

@@ -64,9 +64,11 @@
 #define KVM_INST_MTSRIN		0x7c0001e4
 
 static bool kvm_patching_worked = true;
-char kvm_tmp[1024 * 1024];
 static int kvm_tmp_index;
 
+char kvm_tmp[1024 * 1024];
+void *bss_hole_start, *bss_hole_end;
+
 static inline void kvm_patch_ins(u32 *inst, u32 new_inst)
 {
 	*inst = new_inst;

@@ -707,6 +709,10 @@ static __init void kvm_free_tmp(void)
 	 */
 	kmemleak_free_part(&kvm_tmp[kvm_tmp_index],
 			   ARRAY_SIZE(kvm_tmp) - kvm_tmp_index);
+
+	bss_hole_start = &kvm_tmp[kvm_tmp_index];
+	bss_hole_end = &kvm_tmp[ARRAY_SIZE(kvm_tmp)];
+
 	free_reserved_area(&kvm_tmp[kvm_tmp_index],
 			   &kvm_tmp[ARRAY_SIZE(kvm_tmp)], -1, NULL);
 }

diff --git a/include/asm-generic/sections.h b/include/asm-generic/sections.h
index d1779d442aa5..28a7a56e7c8a 100644
--- a/include/asm-generic/sections.h
+++ b/include/asm-generic/sections.h

@@ -91,6 +91,13 @@ static inline int arch_is_kernel_initmem_freed(unsigned long addr)
 }
 #endif
 
+#ifndef arch_is_bss_hole
+static inline bool arch_is_bss_hole(unsigned long addr)
+{
+	return false;
+}
+#endif
+
 /**
  * memory_contains - checks if an object is contained within a memory region
  * @begin: virtual address of the beginning of the memory region

diff --git a/kernel/locking/lockdep.c b/kernel/locking/lockdep.c
index 4861cf8e274b..cd75b51f15ce 100644
--- a/kernel/locking/lockdep.c
+++ b/kernel/locking/lockdep.c

@@ -675,6 +675,9 @@ static int static_obj(const void *obj)
 	if (arch_is_kernel_initmem_freed(addr))
 		return 0;
 
+	if (arch_is_bss_hole(addr))
+		return 0;
+
 	/*
 	 * static variable?
 	 */

-- 
1.8.3.1

Hi Qian,

Sorry I haven't replied sooner, I've been travelling.

Qian Cai [off-list ref] writes:

The commit 108c14858b9e ("locking/lockdep: Add support for dynamic
keys") introduced a boot warning on powerpc below, because since the
commit 2d4f567103ff ("KVM: PPC: Introduce kvm_tmp framework") adds
kvm_tmp[] into the .bss section and then free the rest of unused spaces
back to the page allocator.

Thanks for debugging this, but I'd like to fix it differently.

kvm_tmp has caused trouble before, with kmemleak, and it can also cause
trouble with STRICT_KERNEL_RWX, so I'd like to change how it's done,
rather than doing more hacks for it.

It should just be a page in text that we use if needed, and don't free,
which should avoid all these problems.

I'll try and get that done and posted soon.

cheers

Hi Qian,

Thank you for the patch! Yet something to improve:

[auto build test ERROR on linus/master]
[cannot apply to v5.3-rc8 next-20190904]
[if your patch is applied to the wrong git tree, please drop us a note to help improve the system]

url:    https://github.com/0day-ci/linux/commits/Qian-Cai/powerpc-lockdep-fix-a-false-positive-warning/20190910-053719
config: powerpc-ps3_defconfig (attached as .config)
compiler: powerpc64-linux-gcc (GCC) 7.4.0
reproduce:
        wget https://raw.githubusercontent.com/intel/lkp-tests/master/sbin/make.cross -O ~/bin/make.cross
        chmod +x ~/bin/make.cross
        # save the attached .config to linux build tree
        GCC_VERSION=7.4.0 make.cross ARCH=powerpc 

If you fix the issue, kindly add following tag
Reported-by: kbuild test robot <redacted>

All errors (new ones prefixed by >>):

quoted

kernel/locking/lockdep.o:(.toc+0x0): undefined reference to `bss_hole_start'
kernel/locking/lockdep.o:(.toc+0x8): undefined reference to `bss_hole_end'

---
0-DAY kernel test infrastructure                Open Source Technology Center
https://lists.01.org/pipermail/kbuild-all                   Intel Corporation