Re: [PATCH v1] mm/madvise: report SIGBUS as -EFAULT for MADV_POPULATE_(READ|WRITE)

[David Hildenbrand]

Hi Andrew,

sorry for not CCing you, absolutely no clue why I accidentally dropped 
you. Can you give this patch a churn? It would be great if we could get 
that into 5.14, so we don't have to deal with differing behavior between 
Linux versions.

Cheers!

On 26.07.21 17:49, David Hildenbrand wrote:
> Doing some extended tests and polishing the man page update for
> MADV_POPULATE_(READ|WRITE), I realized that we end up converting also
> SIGBUS (via -EFAULT) to -EINVAL, making it look like yet another
> madvise() user error.
>
> We want to report only problematic mappings and permission problems that
> the user could have know as -EINVAL.
>
> Let's not convert -EFAULT arising due to SIGBUS (or SIGSEGV) to
> -EINVAL, but instead indicate -EFAULT to user space. While we could also
> convert it to -ENOMEM, using -EFAULT looks more helpful when user space
> might want to troubleshoot what's going wrong: MADV_POPULATE_(READ|WRITE)
> is not part of an final Linux release and we can still adjust the behavior.
>
> Fixes: 4ca9b3859dac ("mm/madvise: introduce MADV_POPULATE_(READ|WRITE) to prefault page tables")
> Cc: Arnd Bergmann <arnd@xxxxxxxx>
> Cc: Michal Hocko <mhocko@xxxxxxxx>
> Cc: Oscar Salvador <osalvador@xxxxxxx>
> Cc: Matthew Wilcox (Oracle) <willy@xxxxxxxxxxxxx>
> Cc: Andrea Arcangeli <aarcange@xxxxxxxxxx>
> Cc: Minchan Kim <minchan@xxxxxxxxxx>
> Cc: Jann Horn <jannh@xxxxxxxxxx>
> Cc: Jason Gunthorpe <jgg@xxxxxxxx>
> Cc: Dave Hansen <dave.hansen@xxxxxxxxx>
> Cc: Hugh Dickins <hughd@xxxxxxxxxx>
> Cc: Rik van Riel <riel@xxxxxxxxxxx>
> Cc: Michael S. Tsirkin <mst@xxxxxxxxxx>
> Cc: Kirill A. Shutemov <kirill.shutemov@xxxxxxxxxxxxxxx>
> Cc: Vlastimil Babka <vbabka@xxxxxxx>
> Cc: Richard Henderson <rth@xxxxxxxxxxx>
> Cc: Ivan Kokshaysky <ink@xxxxxxxxxxxxxxxxxxxx>
> Cc: Matt Turner <mattst88@xxxxxxxxx>
> Cc: Thomas Bogendoerfer <tsbogend@xxxxxxxxxxxxxxxx>
> Cc: "James E.J. Bottomley" <James.Bottomley@xxxxxxxxxxxxxxxxxxxxx>
> Cc: Helge Deller <deller@xxxxxx>
> Cc: Chris Zankel <chris@xxxxxxxxxx>
> Cc: Max Filippov <jcmvbkbc@xxxxxxxxx>
> Cc: Mike Kravetz <mike.kravetz@xxxxxxxxxx>
> Cc: Peter Xu <peterx@xxxxxxxxxx>
> Cc: Rolf Eike Beer <eike-kernel@xxxxxxxxx>
> Cc: Ram Pai <linuxram@xxxxxxxxxx>
> Cc: Shuah Khan <shuah@xxxxxxxxxx>
> Signed-off-by: David Hildenbrand <david@xxxxxxxxxx>
> ---
>   mm/gup.c     | 7 +++++--
>   mm/madvise.c | 4 +++-
>   2 files changed, 8 insertions(+), 3 deletions(-)
>
> diff --git a/mm/gup.c b/mm/gup.c
> index 42b8b1fa6521..b94717977d17 100644
> --- a/mm/gup.c
> +++ b/mm/gup.c
> @@ -1558,9 +1558,12 @@ long faultin_vma_page_range(struct vm_area_struct *vma, unsigned long start,
>   		gup_flags |= FOLL_WRITE;
>   
>   	/*
> -	 * See check_vma_flags(): Will return -EFAULT on incompatible mappings
> -	 * or with insufficient permissions.
> +	 * We want to report -EINVAL instead of -EFAULT for any permission
> +	 * problems or incompatible mappings.
>   	 */
> +	if (check_vma_flags(vma, gup_flags))
> +		return -EINVAL;
> +
>   	return __get_user_pages(mm, start, nr_pages, gup_flags,
>   				NULL, NULL, locked);
>   }
> diff --git a/mm/madvise.c b/mm/madvise.c
> index 6d3d348b17f4..5c065bc8b5f6 100644
> --- a/mm/madvise.c
> +++ b/mm/madvise.c
> @@ -862,10 +862,12 @@ static long madvise_populate(struct vm_area_struct *vma,
>   			switch (pages) {
>   			case -EINTR:
>   				return -EINTR;
> -			case -EFAULT: /* Incompatible mappings / permissions. */
> +			case -EINVAL: /* Incompatible mappings / permissions. */
>   				return -EINVAL;
>   			case -EHWPOISON:
>   				return -EHWPOISON;
> +			case -EFAULT: /* VM_FAULT_SIGBUS or VM_FAULT_SIGSEGV */
> +				return -EFAULT;
>   			default:
>   				pr_warn_once("%s: unhandled return value: %ld\n",
>   					     __func__, pages);


--
Thanks,

David / dhildenb