Re: [PATCH] proc: Fix integer overflow of VmLib

[Michal Hocko <mhocko@kernel.org>]

I guess you meant s@overflow@underflow@ right?

On Thu 05-01-17 00:29:18, Richard Weinberger wrote:
> /proc/<pid>/status can report extremely high VmLib values which
> will confuse monitoring tools.
> VmLib is mm->exec_vm minus text size, where exec_vm is the number of
> bytes backed by an executable memory mapping and text size is
> mm->end_code - mm->start_code as set up by binfmt.
> 
> For the vast majority of all programs text size is smaller than exec_vm.
> But if a program interprets binaries on its own the calculation result
> can be negative.
> UserModeLinux is such an example. It installs and removes lots of PROT_EXEC
> mappings but mm->start_code and mm->start_code remain and VmLib turns
> negative.
> 
> Fix this by detecting the overflow and just return 0.
> For interpreting the value reported by VmLib is anyway useless but
> returning 0 does at least not confuse userspace.

Is really 0 what the userspace expects? Why shouldn't we just report
exec_vm unconditionally? Btw. we used to do something that many years
back https://lkml.org/lkml/2004/8/24/47. We are exporting the text size
so the calculation can be done by the userspace.
 
> Signed-off-by: Richard Weinberger <richard@nod.at>
> ---
>  fs/proc/task_mmu.c | 2 ++
>  1 file changed, 2 insertions(+)
> 
> diff --git a/fs/proc/task_mmu.c b/fs/proc/task_mmu.c
> index 8f96a49178d0..220091c29aa6 100644
> --- a/fs/proc/task_mmu.c
> +++ b/fs/proc/task_mmu.c
> @@ -46,6 +46,8 @@ void task_mem(struct seq_file *m, struct mm_struct *mm)
>  
>  	text = (PAGE_ALIGN(mm->end_code) - (mm->start_code & PAGE_MASK)) >> 10;
>  	lib = (mm->exec_vm << (PAGE_SHIFT-10)) - text;
> +	if ((long)lib < 0)
> +		lib = 0;
>  	swap = get_mm_counter(mm, MM_SWAPENTS);
>  	ptes = PTRS_PER_PTE * sizeof(pte_t) * atomic_long_read(&mm->nr_ptes);
>  	pmds = PTRS_PER_PMD * sizeof(pmd_t) * mm_nr_pmds(mm);
> -- 
> 2.10.2
> 

-- 
Michal Hocko
SUSE Labs