Re: [PATCH 1/3] mm: Make generic arch_is_kernel_initmem_freed() do what it says

Le 27/09/2021 à 15:11, Michael Ellerman a écrit :

Christophe Leroy [off-list ref] writes:

quoted

Commit 7a5da02de8d6 ("locking/lockdep: check for freed initmem in
static_obj()") added arch_is_kernel_initmem_freed() which is supposed
to report whether an object is part of already freed init memory.

For the time being, the generic version of arch_is_kernel_initmem_freed()
always reports 'false', allthough free_initmem() is generically called
on all architectures.

Therefore, change the generic version of arch_is_kernel_initmem_freed()
to check whether free_initmem() has been called. If so, then check
if a given address falls into init memory.

In order to use function init_section_contains(), the fonction is
moved at the end of asm-generic/section.h

Cc: Gerald Schaefer <gerald.schaefer@linux.ibm.com>
Signed-off-by: Christophe Leroy <redacted>
---
  include/asm-generic/sections.h | 31 +++++++++++++++++--------------
  1 file changed, 17 insertions(+), 14 deletions(-)

diff --git a/include/asm-generic/sections.h b/include/asm-generic/sections.h
index d16302d3eb59..d1e5bb2c6b72 100644
--- a/include/asm-generic/sections.h
+++ b/include/asm-generic/sections.h

@@ -172,4 +158,21 @@ static inline bool is_kernel_rodata(unsigned long addr)
  	       addr < (unsigned long)__end_rodata;
  }
  
+/*
+ * Check if an address is part of freed initmem. This is needed on architectures
+ * with virt == phys kernel mapping, for code that wants to check if an address
+ * is part of a static object within [_stext, _end]. After initmem is freed,
+ * memory can be allocated from it, and such allocations would then have
+ * addresses within the range [_stext, _end].
+ */
+#ifndef arch_is_kernel_initmem_freed
+static inline int arch_is_kernel_initmem_freed(unsigned long addr)
+{
+	if (system_state < SYSTEM_RUNNING)
+		return 0;
+
+	return init_section_contains((void *)addr, 1);
+}
+#endif

This will return an incorrect result for a short period during boot
won't it?

See init/main.c:

static int __ref kernel_init(void *unused)
{
	...
	free_initmem();			<- memory is freed here
	mark_readonly();

	/*
	 * Kernel mappings are now finalized - update the userspace page-table
	 * to finalize PTI.
	 */
	pti_finalize();

	system_state = SYSTEM_RUNNING;


After free_initmem() we have address ranges that are now freed initmem,
but arch_is_kernel_initmem_freed() continues to return 0 (false) for all
addresses, until we update system_state.

Possibly that doesn't matter for any of the current callers, but it
seems pretty dicey to me.

Yes I saw it but as function core_kernel_text() uses that criteria for 
deciding whether a given init text address is valid or not, I thought it 
was just ok.

Should we add an intermediate state called for exemple 
SYSTEM_FREEING_INIT just before SYSTEM_RUNNING ?

Christophe