2023-04-21 19:04:14

by Alexey Dobriyan

Subject: [PATCH] ELF: use __builtin_mul_overflow() more

__builtin_mul_overflow() can do multiplication and overflow check
in one line.

Signed-off-by: Alexey Dobriyan <[email protected]>
---

fs/binfmt_elf.c | 3 +--
1 file changed, 1 insertion(+), 2 deletions(-)

--- a/fs/binfmt_elf.c
+++ b/fs/binfmt_elf.c
@@ -1651,9 +1651,8 @@ static int fill_files_note(struct memelfnote *note, struct coredump_params *cprm

/* *Estimated* file count and total data size needed */
count = cprm->vma_count;
- if (count > UINT_MAX / 64)
+ if (__builtin_mul_overflow(count, 64, &size))
return -EINVAL;
- size = count * 64;

names_ofs = (2 + 3 * count) * sizeof(data[0]);
alloc:

2023-04-21 19:47:37

by Andrew Morton

Subject: Re: [PATCH] ELF: use __builtin_mul_overflow() more

On Fri, 21 Apr 2023 21:54:36 +0300 Alexey Dobriyan <[email protected]> wrote:

> __builtin_mul_overflow() can do multiplication and overflow check
> in one line.
>
> --- a/fs/binfmt_elf.c
> +++ b/fs/binfmt_elf.c
> @@ -1651,9 +1651,8 @@ static int fill_files_note(struct memelfnote *note, struct coredump_params *cprm
>
> /* *Estimated* file count and total data size needed */
> count = cprm->vma_count;
> - if (count > UINT_MAX / 64)
> + if (__builtin_mul_overflow(count, 64, &size))
> return -EINVAL;
> - size = count * 64;

Huh, what the heck is that ;)


include/linux/overflow.h has check_mul_overflow() for us to use here.


tools/lib/bpf/libbpf_internal.h uses

#if __has_builtin(__builtin_mul_overflow)

but check_mul_overflow() didn't bother testing for availability.
Probably tools/lib/bpf/libbpf_internal.h should just use
check_mul_overflow().

2023-04-22 09:58:07

by Alexey Dobriyan

Subject: Re: [PATCH] ELF: use __builtin_mul_overflow() more

On Fri, Apr 21, 2023 at 12:39:11PM -0700, Andrew Morton wrote:
> On Fri, 21 Apr 2023 21:54:36 +0300 Alexey Dobriyan <[email protected]> wrote:
>
> > __builtin_mul_overflow() can do multiplication and overflow check
> > in one line.
> >
> > --- a/fs/binfmt_elf.c
> > +++ b/fs/binfmt_elf.c
> > @@ -1651,9 +1651,8 @@ static int fill_files_note(struct memelfnote *note, struct coredump_params *cprm
> >
> > /* *Estimated* file count and total data size needed */
> > count = cprm->vma_count;
> > - if (count > UINT_MAX / 64)
> > + if (__builtin_mul_overflow(count, 64, &size))
> > return -EINVAL;
> > - size = count * 64;
>
> Huh, what the heck is that ;)
>
>
> include/linux/overflow.h has check_mul_overflow() for us to use here.

Oh, no, wrappers.

> tools/lib/bpf/libbpf_internal.h uses
>
> #if __has_builtin(__builtin_mul_overflow)
>
> but check_mul_overflow() didn't bother testing for availability.

gcc 5.1 has __builtin_mul_overflow()

> Probably tools/lib/bpf/libbpf_internal.h should just use
> check_mul_overflow().

I don't know, this is userspace stuff.