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Tue, 1 Sep 2026 10:42:36 +0000 (GMT) Received: from osiris (unknown [9.111.45.16]) by smtpav02.fra02v.mail.ibm.com (Postfix) with ESMTPS; Tue, 1 Sep 2026 10:42:36 +0000 (GMT) Date: Tue, 1 Sep 2026 12:42:34 +0200 From: Heiko Carstens To: Aleksei Nikiforov Cc: linux-s390@vger.kernel.org, Vasily Gorbik , Alexander Gordeev , Ilya Leoshkevich , Juergen Christ Subject: Re: [PATCH] kmsan: fix false warnings in return_address on s390 Message-ID: <20260901104234.14482A96-hca@linux.ibm.com> References: <20260831152811.3817938-2-aleksei.nikiforov@linux.ibm.com> Precedence: bulk X-Mailing-List: linux-s390@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20260831152811.3817938-2-aleksei.nikiforov@linux.ibm.com> X-TM-AS-GCONF: 00 X-Proofpoint-GUID: EiI9YziBIQTOuelj0Vodgk4ceKJg63FV X-Proofpoint-Spam-Details-Enc: AW1haW4tMjYwOTAxMDA5MyBTYWx0ZWRfX+swLFc9H7UU/ s+XDXa6z9GBoymErKM1tXBFoFilBoE+3vgzsSgioX4i0Cwg07rDdqVIHdPGN2pR2EF0YPCzBfH6 pY2CJCGQrXck0sAO3kKlNNivZ2Tx5VWbaGhbVoyytFnIEzIhhOB22XEDhHMXmOQkHYaU34+4Qm/ 68uuaQaO/IilFkaUf6i/1JRazuyrqzQyk1C/PSqS+eetKwBZDUsGteOHZVutXE7869i1AGKkkzx OwT4/gUdTPsLCZLZlENI1q7IABnLLbwv7sIxWwKfVHO5vP1ezUljsQKvcrGWz7WHlKx8v7KU7hO e8Cd7GtAfGG0edD9qOdWAX9p/DrCIVpkvb1iABH9UTsytBT2oMd2ePnNeV+B8iFTWv41UYnqor1 DtqAzv7+0cTzaGsldYrqBOq6Frf0sKe/3cbWk7QRLM/5tgb1NodeMOjK+DAX6aykfBQVAlBvrFf HN/o589JX3klc9ivosw== X-Authority-Analysis: v=2.4 cv=Osl/DS/t c=1 sm=1 tr=0 ts=6a96ac21 cx=c_pps a=bLidbwmWQ0KltjZqbj+ezA==:117 a=bLidbwmWQ0KltjZqbj+ezA==:17 a=kj9zAlcOel0A:10 a=VdqzKS8jKosA:10 a=VkNPw1HP01LnGYTKEx00:22 a=RnoormkPH1_aCDwRdu11:22 a=Y2IxJ9c9Rs8Kov3niI8_:22 a=VnNF1IyMAAAA:8 a=pAONFi0PjJ7zOFmF0UAA:9 a=CjuIK1q_8ugA:10 X-Proofpoint-Spam-Info: AW1haW4tMjYwOTAxMDA5MyBTYWx0ZWRfX3ljG6ga3gaLi KqbDPvvHcjlVMxsBjWg9UnPRpfzEJ/JV3oAGnYIoQKbf6Wz+0mwUikhB2DvLBk4ty4KEkFqQIVX TrdtDs7fg7txLrpDdWA9ScWLt0DfMkU= X-Proofpoint-ORIG-GUID: EiI9YziBIQTOuelj0Vodgk4ceKJg63FV X-Proofpoint-Virus-Version: vendor=baseguard engine=ICAP:2.0.293,Aquarius:18.0.1176,Hydra:6.1.134,FMLib:17.12.100.49 definitions=2026-09-01_03,2026-08-31_01,2025-10-01_01 X-Proofpoint-Spam-Details: rule=outbound_notspam policy=outbound score=0 spamscore=0 malwarescore=0 suspectscore=0 bulkscore=0 lowpriorityscore=0 adultscore=0 impostorscore=0 phishscore=0 clxscore=1015 priorityscore=1501 classifier=typeunknown authscore=0 authtc= authcc= route=outbound adjust=0 reason=mlx scancount=1 engine=8.22.0-2606150000 definitions=main-2609010093 On Mon, Aug 31, 2026 at 05:28:12PM +0200, Aleksei Nikiforov wrote: > Function return_address manually traverses stack frames > using backchain on s390. > Stack frames are written to stack in function prologues > and are not marked as written for kmsan. > This may lead to previous mark being used. > > Let's assume that there was an unitialized area on stack. > Later backchain and other information is written there > in function prologue. > kmsan markings are unchanged in prologue. > That means data is actually initialized but incorrectly marked. > And when data is read, a false warning is emitted. > > It might be possible to fix marking data written in prologue of function, > but it may be somewhat complex, especially in leaf functions > where no stack for next function call is allocated yet > and new function call would be required for kmsan helper function. > > Other approach is to mark return_address function as noinstr > to skip kmsan checks there. That's approach in this patch. > > When kmsan is enabled, a special noinstr wrapper function is used. > Since one more function call is introduced, > depth n of backtrace is incremented to account for this function call. ... > Signed-off-by: Aleksei Nikiforov > --- > arch/s390/include/asm/ftrace.h | 6 ++++++ > arch/s390/kernel/stacktrace.c | 8 ++++++++ > 2 files changed, 14 insertions(+) ... > + > +#ifdef CONFIG_KMSAN > +unsigned long return_address_noinstr(unsigned int n); > +#define ftrace_return_address(n) return_address_noinstr(n) > +#else > #define ftrace_return_address(n) return_address(n) > +#endif ... > +#ifdef CONFIG_KMSAN > +noinstr unsigned long return_address_noinstr(unsigned int n) > +{ > + /* Add 1 to account for call of uninlined function return_address_noinstr */ > + return return_address(n+1); > +} > +#endif Thinking about this again. Wouldn't READ_ONCE_NOCHECK() solve this too? Otherwise I guess we have the same problem in the normal stack unwinder too. Something like this: diff --git a/arch/s390/include/asm/ftrace.h b/arch/s390/include/asm/ftrace.h index 692c484ec163..4a2c46341b86 100644 --- a/arch/s390/include/asm/ftrace.h +++ b/arch/s390/include/asm/ftrace.h @@ -17,11 +17,11 @@ static __always_inline unsigned long return_address(unsigned int n) sf = (struct stack_frame *)current_frame_address(); do { - sf = (struct stack_frame *)sf->back_chain; + sf = (struct stack_frame *)READ_ONCE_NOCHECK(sf->back_chain); if (!sf) return 0; } while (--n); - return sf->gprs[8]; + return READ_ONCE_NOCHECK(sf->gprs[8]); } #define ftrace_return_address(n) return_address(n) But that might also generate an additional stack frame due to an out-of-line call. If that doesn't help we _may_ need to come up with an inline assembly which hides the memory access. E.g. something like this: static __always_inline long ___read(void *ptr) { long val; asm volatile( " lg %[val],0(%[ptr])" : [val] "=d" (val) : [ptr] "a" (ptr)); return val; } and use ___read() instead of READ_ONCE_NOCHECK() within return_address(). Opinions?