From mboxrd@z Thu Jan 1 00:00:00 1970 From: ebiederm@xmission.com (Eric W. Biederman) Subject: Re: [GIT PULL] siginfo fix for v4.16-rc5 Date: Tue, 03 Apr 2018 09:27:18 -0500 Message-ID: <87in98xt4p.fsf@xmission.com> References: <87woypy8zc.fsf@xmission.com> <20180331105658.GA4332@asgard.redhat.com> <87woxpz7k9.fsf@xmission.com> Mime-Version: 1.0 Content-Type: text/plain Return-path: In-Reply-To: (Geert Uytterhoeven's message of "Tue, 3 Apr 2018 09:30:36 +0200") Sender: linux-kernel-owner@vger.kernel.org To: Geert Uytterhoeven Cc: Eugene Syromiatnikov , Linus Torvalds , Linux Kernel Mailing List , Linux/m68k List-Id: linux-m68k@vger.kernel.org Geert Uytterhoeven writes: > Hi Eric, > > On Mon, Apr 2, 2018 at 10:17 PM, Eric W. Biederman > wrote: >> Eugene Syromiatnikov writes: >> >>> So, the offset of the si_lower field is 20 at the current HEAD and was 18 at >>> commits v4.16-rc3~17^2 and v4.16-rc1~159^2~20. I believe this is due to >>> the fact that m68k uses 2-byte default alignment and not 4-byte. >> >> A 2-byte alignment for 4 byte pointers. That is a new one to me. > > Not just for pointers, also for int and long. > And m68k is not the only architecture having such alignment rules. The smallest I have seen previously has been 64bit integers having 32bit alignment. 32bit entities having only 16bit alignment on a 32bit arch was simply a surprise. Even when it works there tend to be good reasons not to do that by default. >> Euguene can you test the patch below. It should be fully robust against >> this kind of craziness. It certainly passes my BUILD_BUG_ON tests for >> m68k. >> >> Eric >> >> From: "Eric W. Biederman" >> Date: Mon, 2 Apr 2018 14:45:42 -0500 >> Subject: [PATCH] signal: Correct the offset of si_pkey and si_lower in struct siginfo on m68k >> >> The change moving addr_lsb into the _sigfault union failed to take >> into account that _sigfault._addr_bnd._lower being a pointer forced >> the entire union to have pointer alignment. The fix for >> _sigfault._addr_bnd._lower having pointer alignment failed to take >> into account that m68k has a pointer alignment less than the size >> of a pointer. So simply making the padding members pointers changed >> the location of later members in the structure. >> >> Fix this by directly computing the needed size of the padding members, >> and making the padding members char arrays of the needed size. AKA >> if __alignof__(void *) is 1 sizeof(short) otherwise __alignof__(void *). >> Which should be exactly the same rules the compiler whould have >> used when computing the padding. > > __alignof__(void *) is 2 not 1 on m68k. I was not expecting __alignof__(void *) to be 1 on m68k. I was testing for anything crazier than m68k. Since there used to be a short in the hole. If your alignment is less than sizeof(short) aka 2 we do need two bytes of pad in there. >> I have tested this change by adding BUILD_BUG_ONs to m68k to verify >> the offset of every member of struct siginfo, and with those testing >> that the offsets of the fields in struct siginfo is the same before >> I changed the generic _sigfault member and after the correction >> to the _sigfault member. >> >> I have also verified that the x86 with it's own BUILD_BUG_ONs to verify >> the offsets of the siginfo members also compiles cleanly. >> >> Cc: stable@vger.kernel.org >> Reported-by: Eugene Syromiatnikov >> Fixes: 859d880cf544 ("signal: Correct the offset of si_pkey in struct siginfo") >> Fixes: b68a68d3dcc1 ("signal: Move addr_lsb into the _sigfault union for clarity") >> Signed-off-by: "Eric W. Biederman" >> --- >> include/linux/compat.h | 6 ++++-- >> include/uapi/asm-generic/siginfo.h | 7 +++++-- >> 2 files changed, 9 insertions(+), 4 deletions(-) >> >> diff --git a/include/linux/compat.h b/include/linux/compat.h >> index e16d07eb08cf..d770e62632d7 100644 >> --- a/include/linux/compat.h >> +++ b/include/linux/compat.h >> @@ -221,6 +221,8 @@ typedef struct compat_siginfo { >> #ifdef __ARCH_SI_TRAPNO >> int _trapno; /* TRAP # which caused the signal */ >> #endif >> +#define __COMPAT_ADDR_BND_PKEY_PAD (__alignof__(compat_uptr_t) < sizeof(short) ? \ >> + sizeof(short) : __alignof__(compat_uptr_t)) > > On m68k, __alignof__(compat_uptr_t) == 2, so it will use > __alignof__(compat_uptr_t) padding bytes. > > Note that while the test is wrong, the end result is correct :-) > > Hence you could just use __alignof__(compat_uptr_t) padding bytes > unconditionally? Unless there is something crazier than m68k that only needs 1 byte alignment for pointers. In which case this code really needs 2 padding bytes to avoid introducing a regression there as historically there was a short in the padding hole. So I don't see anything wrong with the test. >> union { >> /* >> * used when si_code=BUS_MCEERR_AR or >> @@ -229,13 +231,13 @@ typedef struct compat_siginfo { >> short int _addr_lsb; /* Valid LSB of the reported address. */ >> /* used when si_code=SEGV_BNDERR */ >> struct { >> - compat_uptr_t _dummy_bnd; >> + char _dummy_bnd[__COMPAT_ADDR_BND_PKEY_PAD]; >> compat_uptr_t _lower; >> compat_uptr_t _upper; >> } _addr_bnd; >> /* used when si_code=SEGV_PKUERR */ >> struct { >> - compat_uptr_t _dummy_pkey; >> + char _dummy_pkey[__COMPAT_ADDR_BND_PKEY_PAD]; >> u32 _pkey; >> } _addr_pkey; >> }; >> diff --git a/include/uapi/asm-generic/siginfo.h b/include/uapi/asm-generic/siginfo.h >> index 4b3520bf67ba..6d789648473d 100644 >> --- a/include/uapi/asm-generic/siginfo.h >> +++ b/include/uapi/asm-generic/siginfo.h >> @@ -94,6 +94,9 @@ typedef struct siginfo { >> unsigned int _flags; /* see ia64 si_flags */ >> unsigned long _isr; /* isr */ >> #endif >> + >> +#define __ADDR_BND_PKEY_PAD (__alignof__(void *) < sizeof(short) ? \ >> + sizeof(short) : __alignof__(void *)) > > Likewise. > >> union { >> /* >> * used when si_code=BUS_MCEERR_AR or >> @@ -102,13 +105,13 @@ typedef struct siginfo { >> short _addr_lsb; /* LSB of the reported address */ >> /* used when si_code=SEGV_BNDERR */ >> struct { >> - void *_dummy_bnd; >> + char _dummy_bnd[__ADDR_BND_PKEY_PAD]; >> void __user *_lower; >> void __user *_upper; >> } _addr_bnd; >> /* used when si_code=SEGV_PKUERR */ >> struct { >> - void *_dummy_pkey; >> + char _dummy_pkey[__ADDR_BND_PKEY_PAD]; >> __u32 _pkey; >> } _addr_pkey; >> }; > > Gr{oetje,eeting}s, > > Geert