Verify memory sizes on 32 bit systems.
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Message ID 20180730190146.GA8535@localhost
State New
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Series
  • Verify memory sizes on 32 bit systems.
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Commit Message

Tobias Stoeckmann July 30, 2018, 7:01 p.m. UTC
Large file system support is activated by default, which means that
on 32 bit systems, off_t is 64 bit in size.

Using st.st_size or any other 64 bit variable with mmap can lead to
integer truncation and therefore insufficient memory mapping.

Signed-off-by: Tobias Stoeckmann <tobias@stoeckmann.org>
---
 libkmod/libkmod-file.c | 2 ++
 1 file changed, 2 insertions(+)

Comments

Lucas De Marchi Aug. 3, 2018, 9:07 p.m. UTC | #1
On Mon, Jul 30, 2018 at 12:02 PM Tobias Stoeckmann
<tobias@stoeckmann.org> wrote:
>
> Large file system support is activated by default, which means that
> on 32 bit systems, off_t is 64 bit in size.
>
> Using st.st_size or any other 64 bit variable with mmap can lead to
> integer truncation and therefore insufficient memory mapping.
>
> Signed-off-by: Tobias Stoeckmann <tobias@stoeckmann.org>
> ---
>  libkmod/libkmod-file.c | 2 ++
>  1 file changed, 2 insertions(+)
>
> diff --git a/libkmod/libkmod-file.c b/libkmod/libkmod-file.c
> index 5eeba6a..86f34c6 100644
> --- a/libkmod/libkmod-file.c
> +++ b/libkmod/libkmod-file.c
> @@ -255,6 +255,8 @@ static int load_reg(struct kmod_file *file)
>                 return -errno;
>
>         file->size = st.st_size;
> +       if ((uintmax_t)st.st_size > (uintmax_t)SIZE_MAX)

why casting?

Lucas De Marchi

> +               return -EFBIG;
>         file->memory = mmap(NULL, file->size, PROT_READ, MAP_PRIVATE,
>                             file->fd, 0);
>         if (file->memory == MAP_FAILED)
> --
> 2.13.2
>
> --
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Tobias Stoeckmann Aug. 3, 2018, 9:10 p.m. UTC | #2
On Fri, Aug 03, 2018 at 02:07:34PM -0700, Lucas De Marchi wrote:
> > +       if ((uintmax_t)st.st_size > (uintmax_t)SIZE_MAX)
> 
> why casting?

Without casting, a warning could be triggered on 64 bit systems,
because on these systems a signed 64 bit int (off_t) is compared
with an unsigned 64 bit int (size_t) which could never be larger.

Casting it to uintmax_t, which is at least 64 bits in size,
silences this error.

No technical reason beyond that.


Tobias
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Lucas De Marchi Aug. 3, 2018, 9:33 p.m. UTC | #3
On Fri, Aug 3, 2018 at 2:10 PM Tobias Stoeckmann <tobias@stoeckmann.org> wrote:
>
> On Fri, Aug 03, 2018 at 02:07:34PM -0700, Lucas De Marchi wrote:
> > > +       if ((uintmax_t)st.st_size > (uintmax_t)SIZE_MAX)
> >
> > why casting?
>
> Without casting, a warning could be triggered on 64 bit systems,
> because on these systems a signed 64 bit int (off_t) is compared
> with an unsigned 64 bit int (size_t) which could never be larger.
>
> Casting it to uintmax_t, which is at least 64 bits in size,
> silences this error.

so you are casting the *sign* away, ok. But casting only st.st_size
should suffice.

Lucas De Marchi

>
> No technical reason beyond that.
>
>
> Tobias
Tobias Stoeckmann Aug. 3, 2018, 9:51 p.m. UTC | #4
On Fri, Aug 03, 2018 at 02:33:51PM -0700, Lucas De Marchi wrote:
> so you are casting the *sign* away, ok. But casting only st.st_size
> should suffice.

I've just tried with some examples and I couldn't trigger the
warning which I have seen in other situations like this, i.e.
"always false due to limited range of data type".

If I cast the off_t to size_t, the compiler knows that it is not
possible that it could ever be larger than SIZE_MAX, optimizing
the block away. So I took uintmax_t.

But as long as there's no annoying warning, I am fine with just
casting st_size.


Tobias
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Patch
diff mbox series

diff --git a/libkmod/libkmod-file.c b/libkmod/libkmod-file.c
index 5eeba6a..86f34c6 100644
--- a/libkmod/libkmod-file.c
+++ b/libkmod/libkmod-file.c
@@ -255,6 +255,8 @@  static int load_reg(struct kmod_file *file)
 		return -errno;
 
 	file->size = st.st_size;
+	if ((uintmax_t)st.st_size > (uintmax_t)SIZE_MAX)
+		return -EFBIG;
 	file->memory = mmap(NULL, file->size, PROT_READ, MAP_PRIVATE,
 			    file->fd, 0);
 	if (file->memory == MAP_FAILED)