Date: Tue, 18 Jul 2023 09:49:27 -0400 From: Justin Hibbits <jhibbits@FreeBSD.org> To: Jessica Clarke <jrtc27@freebsd.org> Cc: Ka Ho Ng <khng@FreeBSD.org>, "src-committers@freebsd.org" <src-committers@FreeBSD.org>, "dev-commits-src-all@freebsd.org" <dev-commits-src-all@FreeBSD.org>, "dev-commits-src-main@freebsd.org" <dev-commits-src-main@FreeBSD.org> Subject: Re: git: 034c0856018c - main - modules: fix freebsd32_modstat on big endian platforms Message-ID: <20230718094927.1474f972@ralga.knownspace> In-Reply-To: <EFAA3717-5223-4247-8193-D623741D5A0B@freebsd.org> References: <202307070424.3674OBaa074389@gitrepo.freebsd.org> <EFAA3717-5223-4247-8193-D623741D5A0B@freebsd.org>
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On Fri, 7 Jul 2023 18:59:41 +0100 Jessica Clarke <jrtc27@freebsd.org> wrote: > On 7 Jul 2023, at 05:24, Ka Ho Ng <khng@FreeBSD.org> wrote: > >=20 > > The branch main has been updated by khng: > >=20 > > URL: > > https://cgit.FreeBSD.org/src/commit/?id=3D034c0856018ccf73c0f2d6140825f= 3edf43f47b2 > >=20 > > commit 034c0856018ccf73c0f2d6140825f3edf43f47b2 > > Author: Ka Ho Ng <khng@FreeBSD.org> > > AuthorDate: 2023-07-07 04:21:01 +0000 > > Commit: Ka Ho Ng <khng@FreeBSD.org> > > CommitDate: 2023-07-07 04:22:59 +0000 > >=20 > > modules: fix freebsd32_modstat on big endian platforms > >=20 > > The layout of modspecific_t on both little endian and big endian > > are as follows: > > |0|1|2|3|4|5|6|7| > > +-------+-------+ > > |uintval| | > > +-------+-------+ > > |ulongval | > > +-------+-------+ > >=20 > > For the following code snippet: > > CP(mod->data, data32, longval); > > CP(mod->data, data32, ulongval); > > It only takes care of little endian platforms that it truncates > > the highest 32bit automatically. However on big endian platforms it > > takes the highest 32bit instead. This eventually returns a garbage > > syscall number to the 32bit userland. =20 >=20 > This fixes the case where uintval is the active member, but breaks the > case where ulongval is the active member, since it=E2=80=99ll take bytes = 0, 1, > 2 and 3 which are the high bytes for BE, not the low bytes. This is an > intrinsic issue with BE; LE is very permissive when you access the > wrong union member for different-width integer types, but BE is not > and you have to access the right one. How does userspace know which > member to access, anyway? The kernel needs to repeat that and copy > the right member based on that information, otherwise it is > inherently impossible to have both cases work on BE. >=20 > > Since modspecific_t's usage currently is for the use of syscall > > modules, we only initialize modspecific32_t with uintval. =20 >=20 > Is this saying that uintval is always the active member, never > ulongval? If so, there need to be comments, both in freebsd32_modstat > to justify this and on the type to say =E2=80=9CDO NOT USE (u)longval=E2= =80=9D, > otherwise there is a significant risk someone will start using > (u)longval and we=E2=80=99ll be stuck. Perhaps (u)longval should even be > renamed to be ABI-compatibility padding (and aligning). >=20 > Jess A better solution is to make these fields fixed size, so we don't run into this problem again. Is this feasible, factoring in any other consumers? - Justin >=20 > > Now on both BE and LE > > 64-bit platforms it always pick up the first 4 bytes. > >=20 > > Sponsored by: Juniper Networks, Inc. > > Reviewed by: markj > > Differential Revision: https://reviews.freebsd.org/D40814 > > MFC after: 1 week > > --- > > sys/kern/kern_module.c | 5 ++--- > > 1 file changed, 2 insertions(+), 3 deletions(-) > >=20 > > diff --git a/sys/kern/kern_module.c b/sys/kern/kern_module.c > > index 40e47868481c..d4edc64a5ce4 100644 > > --- a/sys/kern/kern_module.c > > +++ b/sys/kern/kern_module.c > > @@ -520,10 +520,9 @@ freebsd32_modstat(struct thread *td, struct > > freebsd32_modstat_args *uap) id =3D mod->id; > > refs =3D mod->refs; > > name =3D mod->name; > > - CP(mod->data, data32, intval); > > + _Static_assert(sizeof(data32) =3D=3D sizeof(data32.uintval), > > + "bad modspecific32_t size"); > > CP(mod->data, data32, uintval); > > - CP(mod->data, data32, longval); > > - CP(mod->data, data32, ulongval); > > MOD_SUNLOCK; > > stat32 =3D uap->stat; > > =20
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