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

[Christophe Leroy]

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 ?

> 
> Signed-off-by: Rick Edgecombe <rick.p.edgecombe@xxxxxxxxx>
> Acked-by: Michael Ellerman <mpe@xxxxxxxxxxxxxx>
> Cc: Michael Ellerman <mpe@xxxxxxxxxxxxxx>
> Cc: Nicholas Piggin <npiggin@xxxxxxxxx>
> Cc: Christophe Leroy <christophe.leroy@xxxxxxxxxx>
> Cc: Aneesh Kumar K.V <aneesh.kumar@xxxxxxxxxx>
> Cc: Naveen N. Rao <naveen.n.rao@xxxxxxxxxxxxx>
> Cc: linuxppc-dev@xxxxxxxxxxxxxxxx
> 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@xxxxxxxxx/
> ---
>   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,

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.

> +	};
>   	/*
>   	 * 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

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