linux-kernel - Re: [PATCH] arm64/pageattr: Propagate return value from __change_memory

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Message-ID: <aQn4EwKar66UZ7rz@willie-the-truck>
Date: Tue, 4 Nov 2025 12:56:51 +0000
From: Will Deacon <will@...nel.org>
To: Dev Jain <dev.jain@....com>
Cc: Yang Shi <yang@...amperecomputing.com>, catalin.marinas@....com,
	ryan.roberts@....com, rppt@...nel.org,
	shijie@...amperecomputing.com, linux-arm-kernel@...ts.infradead.org,
	linux-kernel@...r.kernel.org, stable@...r.kernel.org
Subject: Re: [PATCH] arm64/pageattr: Propagate return value from
 __change_memory_common

On Tue, Nov 04, 2025 at 09:06:12AM +0530, Dev Jain wrote:
> On 04/11/25 12:15 am, Yang Shi wrote:
> > On 11/3/25 7:16 AM, Will Deacon wrote:
> > > On Mon, Nov 03, 2025 at 11:43:06AM +0530, Dev Jain wrote:
> > > > Post a166563e7ec3 ("arm64: mm: support large block mapping when
> > > > rodata=full"),
> > > > __change_memory_common has a real chance of failing due to split
> > > > failure.
> > > > Before that commit, this line was introduced in c55191e96caa,
> > > > still having
> > > > a chance of failing if it needs to allocate pagetable memory in
> > > > apply_to_page_range, although that has never been observed to be true.
> > > > In general, we should always propagate the return value to the caller.
> > > > 
> > > > Cc: stable@...r.kernel.org
> > > > Fixes: c55191e96caa ("arm64: mm: apply r/o permissions of VM
> > > > areas to its linear alias as well")
> > > > Signed-off-by: Dev Jain <dev.jain@....com>
> > > > ---
> > > > Based on Linux 6.18-rc4.
> > > > 
> > > >   arch/arm64/mm/pageattr.c | 5 ++++-
> > > >   1 file changed, 4 insertions(+), 1 deletion(-)
> > > > 
> > > > diff --git a/arch/arm64/mm/pageattr.c b/arch/arm64/mm/pageattr.c
> > > > index 5135f2d66958..b4ea86cd3a71 100644
> > > > --- a/arch/arm64/mm/pageattr.c
> > > > +++ b/arch/arm64/mm/pageattr.c
> > > > @@ -148,6 +148,7 @@ static int change_memory_common(unsigned
> > > > long addr, int numpages,
> > > >       unsigned long size = PAGE_SIZE * numpages;
> > > >       unsigned long end = start + size;
> > > >       struct vm_struct *area;
> > > > +    int ret;
> > > >       int i;
> > > >         if (!PAGE_ALIGNED(addr)) {
> > > > @@ -185,8 +186,10 @@ static int change_memory_common(unsigned
> > > > long addr, int numpages,
> > > >       if (rodata_full && (pgprot_val(set_mask) == PTE_RDONLY ||
> > > >                   pgprot_val(clear_mask) == PTE_RDONLY)) {
> > > >           for (i = 0; i < area->nr_pages; i++) {
> > > > - __change_memory_common((u64)page_address(area->pages[i]),
> > > > +            ret =
> > > > __change_memory_common((u64)page_address(area->pages[i]),
> > > >                              PAGE_SIZE, set_mask, clear_mask);
> > > > +            if (ret)
> > > > +                return ret;
> > > Hmm, this means we can return failure half-way through the operation. Is
> > > that something callers are expecting to handle? If so, how can they tell
> > > how far we got?
> > 
> > IIUC the callers don't have to know whether it is half-way or not
> > because the callers will change the permission back (e.g. to RW) for the
> > whole range when freeing memory.
> 
> Yes, it is the caller's responsibility to set VM_FLUSH_RESET_PERMS flag.
> Upon vfree(), it will change the direct map permissions back to RW.

Ok, but vfree() ends up using update_range_prot() to do that and if we
need to worry about that failing (as per your commit message), then
we're in trouble because the calls to set_area_direct_map() are unchecked.

In other words, this patch is either not necessary or it is incomplete.

Will