[PATCH v3 07/12] powerpc: Use initializer for struct vm_unmapped_area_info

Future changes will need to add a new member to struct
vm_unmapped_area_info. This would cause trouble for any call site that
doesn't initialize the struct. Currently every caller sets each member
manually, so if new members are added they will be uninitialized and the
core code parsing the struct will see garbage in the new member.

It could be possible to initialize the new member manually to 0 at each
call site. This and a couple other options were discussed, and a working
consensus (see links) was that in general the best way to accomplish this
would be via static initialization with designated member initiators.
Having some struct vm_unmapped_area_info instances not zero initialized
will put those sites at risk of feeding garbage into vm_unmapped_area() if
the convention is to zero initialize the struct and any new member addition
misses a call site that initializes each member manually.

It could be possible to leave the code mostly untouched, and just change
the line:
struct vm_unmapped_area_info info
to:
struct vm_unmapped_area_info info = {};

However, that would leave cleanup for the members that are manually set
to zero, as it would no longer be required.

So to be reduce the chance of bugs via uninitialized members, instead
simply continue the process to initialize the struct this way tree wide.
This will zero any unspecified members. Move the member initializers to the
struct declaration when they are known at that time. Leave the members out
that were manually initialized to zero, as this would be redundant for
designated initializers.

Signed-off-by: Rick Edgecombe <rick.p.edgecombe@intel.com>
Acked-by: Michael Ellerman <mpe@ellerman.id.au>
Cc: Michael Ellerman <mpe@ellerman.id.au>
Cc: Nicholas Piggin <npiggin@gmail.com>
Cc: Christophe Leroy <redacted>
Cc: Aneesh Kumar K.V <aneesh.kumar@kernel.org>
Cc: Naveen N. Rao <redacted>
Cc: linuxppc-dev@lists.ozlabs.org
Link: https://lore.kernel.org/lkml/202402280912.33AEE7A9CF@keescook/#t
Link: https://lore.kernel.org/lkml/j7bfvig3gew3qruouxrh7z7ehjjafrgkbcmg6tcghhfh3rhmzi@wzlcoecgy5rs/
---
v3:
 - Fixed spelling errors in log
 - Be consistent about field vs member in log

Hi,

This patch was split and refactored out of a tree-wide change [0] to just
zero-init each struct vm_unmapped_area_info. The overall goal of the
series is to help shadow stack guard gaps. Currently, there is only one
arch with shadow stacks, but two more are in progress. It is compile tested
only.

There was further discussion that this method of initializing the structs
while nice in some ways has a greater risk of introducing bugs in some of
the more complicated callers. Since this version was reviewed my arch
maintainers already, leave it as was already acknowledged.

Thanks,

Rick

[0] https://lore.kernel.org/lkml/20240226190951.3240433-6-rick.p.edgecombe@intel.com/
---
 arch/powerpc/mm/book3s64/slice.c | 23 ++++++++++++-----------
 1 file changed, 12 insertions(+), 11 deletions(-)

diff --git a/arch/powerpc/mm/book3s64/slice.c b/arch/powerpc/mm/book3s64/slice.c
index c0b58afb9a47..6c7ac8c73a6c 100644
--- a/arch/powerpc/mm/book3s64/slice.c
+++ b/arch/powerpc/mm/book3s64/slice.c

@@ -282,12 +282,12 @@ static unsigned long slice_find_area_bottomup(struct mm_struct *mm,
 {
 	int pshift = max_t(int, mmu_psize_defs[psize].shift, PAGE_SHIFT);
 	unsigned long found, next_end;
-	struct vm_unmapped_area_info info;
-
-	info.flags = 0;
-	info.length = len;
-	info.align_mask = PAGE_MASK & ((1ul << pshift) - 1);
-	info.align_offset = 0;
+	struct vm_unmapped_area_info info = {
+		.flags = 0,
+		.length = len,
+		.align_mask = PAGE_MASK & ((1ul << pshift) - 1),
+		.align_offset = 0
+	};
 	/*
 	 * Check till the allow max value for this mmap request
 	 */

@@ -326,13 +326,14 @@ static unsigned long slice_find_area_topdown(struct mm_struct *mm,
 {
 	int pshift = max_t(int, mmu_psize_defs[psize].shift, PAGE_SHIFT);
 	unsigned long found, prev;
-	struct vm_unmapped_area_info info;
+	struct vm_unmapped_area_info info = {
+		.flags = VM_UNMAPPED_AREA_TOPDOWN,
+		.length = len,
+		.align_mask = PAGE_MASK & ((1ul << pshift) - 1),
+		.align_offset = 0
+	};
 	unsigned long min_addr = max(PAGE_SIZE, mmap_min_addr);
 
-	info.flags = VM_UNMAPPED_AREA_TOPDOWN;
-	info.length = len;
-	info.align_mask = PAGE_MASK & ((1ul << pshift) - 1);
-	info.align_offset = 0;
 	/*
 	 * If we are trying to allocate above DEFAULT_MAP_WINDOW
 	 * Add the different to the mmap_base.

-- 
2.34.1

Le 12/03/2024 à 23:28, Rick Edgecombe a écrit :

Future changes will need to add a new member to struct
vm_unmapped_area_info. This would cause trouble for any call site that
doesn't initialize the struct. Currently every caller sets each member
manually, so if new members are added they will be uninitialized and the
core code parsing the struct will see garbage in the new member.

It could be possible to initialize the new member manually to 0 at each
call site. This and a couple other options were discussed, and a working
consensus (see links) was that in general the best way to accomplish this
would be via static initialization with designated member initiators.
Having some struct vm_unmapped_area_info instances not zero initialized
will put those sites at risk of feeding garbage into vm_unmapped_area() if
the convention is to zero initialize the struct and any new member addition
misses a call site that initializes each member manually.

It could be possible to leave the code mostly untouched, and just change
the line:
struct vm_unmapped_area_info info
to:
struct vm_unmapped_area_info info = {};

However, that would leave cleanup for the members that are manually set
to zero, as it would no longer be required.

So to be reduce the chance of bugs via uninitialized members, instead
simply continue the process to initialize the struct this way tree wide.
This will zero any unspecified members. Move the member initializers to the
struct declaration when they are known at that time. Leave the members out
that were manually initialized to zero, as this would be redundant for
designated initializers.

I understand from this text that, as agreed, this patch removes the 
pointless/redundant zero-init of individual members. But it is not what 
is done, see below ?

Please remove zero-init as agreed and explained in the commit message

+		.length = len,
+		.align_mask = PAGE_MASK & ((1ul << pshift) - 1),
+		.align_offset = 0

Same here.

Same here.

+	};
  	unsigned long min_addr = max(PAGE_SIZE, mmap_min_addr);
  
-	info.flags = VM_UNMAPPED_AREA_TOPDOWN;
-	info.length = len;
-	info.align_mask = PAGE_MASK & ((1ul << pshift) - 1);
-	info.align_offset = 0;
  	/*
  	 * If we are trying to allocate above DEFAULT_MAP_WINDOW
  	 * Add the different to the mmap_base.

Christophe

On Wed, 2024-03-13 at 06:44 +0000, Christophe Leroy wrote:

I understand from this text that, as agreed, this patch removes the 
pointless/redundant zero-init of individual members. But it is not
what 
is done, see below ?

Err, right. I think I decided to leave it because it was already acked
and there wasn't enough discussion on the ack to be sure. I will update
it.

"Edgecombe, Rick P" [off-list ref] writes:

On Wed, 2024-03-13 at 06:44 +0000, Christophe Leroy wrote:

quoted

I understand from this text that, as agreed, this patch removes the 
pointless/redundant zero-init of individual members. But it is not
what 
is done, see below ?

Err, right. I think I decided to leave it because it was already acked
and there wasn't enough discussion on the ack to be sure. I will update
it.

That's fine by me, you can keep my ack.

cheers