linux-kernel - Re: [PATCH v10 3/4] random: introduce generic vDSO getrandom() implementation

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Date:   Wed, 30 Nov 2022 15:59:27 +0100
From:   "Jason A. Donenfeld" <Jason@...c4.com>
To:     Florian Weimer <fweimer@...hat.com>
Cc:     linux-kernel@...r.kernel.org, patches@...ts.linux.dev,
        tglx@...utronix.de, linux-crypto@...r.kernel.org,
        linux-api@...r.kernel.org, x86@...nel.org,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Adhemerval Zanella Netto <adhemerval.zanella@...aro.org>,
        Carlos O'Donell <carlos@...hat.com>,
        Arnd Bergmann <arnd@...db.de>,
        Christian Brauner <brauner@...nel.org>
Subject: Re: [PATCH v10 3/4] random: introduce generic vDSO getrandom()
 implementation

On Wed, Nov 30, 2022 at 03:51:01PM +0100, Jason A. Donenfeld wrote:
> Hi Florian,
> 
> On Wed, Nov 30, 2022 at 11:44:30AM +0100, Florian Weimer wrote:
> > * Jason A. Donenfeld:
> > 
> > > diff --git a/include/vdso/datapage.h b/include/vdso/datapage.h
> > > index 73eb622e7663..9ae4d76b36c7 100644
> > > --- a/include/vdso/datapage.h
> > > +++ b/include/vdso/datapage.h
> > > @@ -109,6 +109,16 @@ struct vdso_data {
> > >  	struct arch_vdso_data	arch_data;
> > >  };
> > >  
> > > +/**
> > > + * struct vdso_rng_data - vdso RNG state information
> > > + * @generation:	a counter representing the number of RNG reseeds
> > > + * @is_ready:	whether the RNG is initialized
> > > + */
> > > +struct vdso_rng_data {
> > > +	unsigned long	generation;
> > > +	bool		is_ready;
> > > +};
> > > +
> > 
> > I don't think you can use a type like long here.  The header says this:
> > 
> >  * vdso_data will be accessed by 64 bit and compat code at the same time
> >  * so we should be careful before modifying this structure.
> > 
> > So the ABI must be same for 32-bit and 64-bit mode, and long isn't.
> 
> Excellent point. The size of the type needs to be explicit. Will fix.

I'll do something like this:

diff --git a/include/vdso/datapage.h b/include/vdso/datapage.h
index 80e25cdcdb1c..218bbeac5613 100644
--- a/include/vdso/datapage.h
+++ b/include/vdso/datapage.h
@@ -19,6 +19,19 @@
 #include <vdso/time32.h>
 #include <vdso/time64.h>

+/**
+ * type vdso_kernel_ulong - unsigned long type that matches kernel's unsigned long
+ *
+ * The structs in this file must operate the same way in both 64-bit code and
+ * in 32-bit compat code, over the same potentially 64-bit kernel. So, this
+ * type represents the size of an unsigned long as used by kernel-space code.
+ */
+#ifdef CONFIG_64BIT
+typedef u64 vdso_kernel_ulong;
+#else
+typedef u32 vdso_kernel_ulong;
+#endif
+
 #ifdef CONFIG_ARCH_HAS_VDSO_DATA
 #include <asm/vdso/data.h>
 #else
@@ -115,8 +128,8 @@ struct vdso_data {
  * @is_ready:	signals whether the RNG is initialized
  */
 struct vdso_rng_data {
-	unsigned long	generation;
-	bool		is_ready;
+	vdso_kernel_ulong	generation;
+	bool			is_ready;
 };

 /*