NetBSD Problem Report #57979

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Date: Sat,  2 Mar 2024 02:31:53 +0000 (UTC)
From: cat@iki.fi
Reply-To: cat@iki.fi
To: gnats-bugs@NetBSD.org
Subject: kern.ipc.shmmax default size, loss of precision in calculation
X-Send-Pr-Version: www-1.0

>Number:         57979
>Category:       kern
>Synopsis:       kern.ipc.shmmax default size, loss of precision in calculation
>Confidential:   no
>Severity:       non-critical
>Priority:       medium
>Responsible:    kern-bug-people
>State:          closed
>Class:          sw-bug
>Submitter-Id:   net
>Arrival-Date:   Sat Mar 02 02:35:00 +0000 2024
>Closed-Date:    Mon Mar 11 22:24:29 +0000 2024
>Last-Modified:  Mon Mar 11 22:24:29 +0000 2024
>Originator:     Patrik Andersin
>Release:        netbsd-10-0-RC5
>Organization:
>Environment:
NetBSD savannah.cat.iki.fi 10.0_RC5 NetBSD 10.0_RC5 (XEN3_DOM0) #7: Sat Mar  2 02:50:54 EET 2024  root@savannah.cat.iki.fi:/usr/obj/sys/arch/amd64/compile/XEN3_DOM0 amd64

>Description:
I stumbled on a problem in calculating default kern.ipc.shmmax value.
Multiplying a uint64_t with uint32_t (PAGE_SIZE) leads to loss of precision.

Setting memory of xen dom0 machine to 16G results in:

# sysctl -a | grep shm
kern.ipc.sysvshm = 1
kern.ipc.shmmax = 0
kern.ipc.shmmni = 128
kern.ipc.shmseg = 128
kern.ipc.shmmaxpgs = 0
kern.ipc.shm_use_phys = 0

With 16GB memory, value of physmem is 4194304. This divided by 4 and multiplied by 4096 is 4294967296 dec and in hex 0x01 00 00 00 00. This gets truncated to 32 bits and result to 0x00.


I also tested this with GENERIC kernel running on xen hvm domu machine.
Setting memory of domu to 16393 (~16GB) results in:

kern.ipc.sysvshm = 1
kern.ipc.shmmax = 159744
kern.ipc.shmmni = 128
kern.ipc.shmseg = 128
kern.ipc.shmmaxpgs = 39
kern.ipc.shm_use_phys = 0



>How-To-Repeat:
Boot xen dom0 machine with following line in /boot.cfg

menu=Xen:load /netbsd-XEN3_DOM0 root=raid0a console=pc;multiboot /xen.gz dom0_mem=16G

>Fix:
Index: sysv_shm.c
===================================================================
RCS file: /cvsroot/src/sys/kern/sysv_shm.c,v
retrieving revision 1.141
diff -u -r1.141 sysv_shm.c
--- sysv_shm.c	9 Oct 2019 17:47:13 -0000	1.141
+++ sysv_shm.c	2 Mar 2024 01:49:21 -0000
@@ -961,7 +961,7 @@
 	    ALIGN(shminfo.shmmni * sizeof(struct shmid_ds)));

 	if (shminfo.shmmax == 0)
-		shminfo.shmmax = uimax(physmem / 4, 1024) * PAGE_SIZE;
+		shminfo.shmmax = uimax(physmem / 4, 1024) * (uint64_t)PAGE_SIZE;
 	else
 		shminfo.shmmax *= PAGE_SIZE;
 	shminfo.shmall = shminfo.shmmax / PAGE_SIZE;

>Release-Note:

>Audit-Trail:
From: "Michael van Elst" <mlelstv@netbsd.org>
To: gnats-bugs@gnats.NetBSD.org
Cc: 
Subject: PR/57979 CVS commit: src/sys/kern
Date: Sat, 2 Mar 2024 08:59:47 +0000

 Module Name:	src
 Committed By:	mlelstv
 Date:		Sat Mar  2 08:59:47 UTC 2024

 Modified Files:
 	src/sys/kern: sysv_shm.c

 Log Message:
 Avoid overflow when computing kern.ipc.shmmax. Keep shmmax (bytes) and
 shmall (pages) values aligned and use arithmetic everywhere instead
 of shifts.
 Should fix PR 57979


 To generate a diff of this commit:
 cvs rdiff -u -r1.141 -r1.142 src/sys/kern/sysv_shm.c

 Please note that diffs are not public domain; they are subject to the
 copyright notices on the relevant files.

State-Changed-From-To: open->needs-pullups
State-Changed-By: riastradh@NetBSD.org
State-Changed-When: Sat, 02 Mar 2024 22:36:16 +0000
State-Changed-Why:
Looks like this should apply cleanly to 10 and 9, and with a small
additional patch to 8.


State-Changed-From-To: needs-pullups->pending-pullups
State-Changed-By: riastradh@NetBSD.org
State-Changed-When: Mon, 11 Mar 2024 01:09:05 +0000
State-Changed-Why:
pullup-10 #626
pullup-9 #1814
pullup-8 #1945


From: "Martin Husemann" <martin@netbsd.org>
To: gnats-bugs@gnats.NetBSD.org
Cc: 
Subject: PR/57979 CVS commit: [netbsd-10] src/sys/kern
Date: Mon, 11 Mar 2024 18:00:06 +0000

 Module Name:	src
 Committed By:	martin
 Date:		Mon Mar 11 18:00:06 UTC 2024

 Modified Files:
 	src/sys/kern [netbsd-10]: sysv_shm.c

 Log Message:
 Pull up following revision(s) (requested by riastradh in ticket #626):

 	sys/kern/sysv_shm.c: revision 1.142

 Avoid overflow when computing kern.ipc.shmmax. Keep shmmax (bytes) and
 shmall (pages) values aligned and use arithmetic everywhere instead
 of shifts.

 Should fix PR 57979


 To generate a diff of this commit:
 cvs rdiff -u -r1.141 -r1.141.26.1 src/sys/kern/sysv_shm.c

 Please note that diffs are not public domain; they are subject to the
 copyright notices on the relevant files.

From: "Martin Husemann" <martin@netbsd.org>
To: gnats-bugs@gnats.NetBSD.org
Cc: 
Subject: PR/57979 CVS commit: [netbsd-9] src/sys/kern
Date: Mon, 11 Mar 2024 18:01:48 +0000

 Module Name:	src
 Committed By:	martin
 Date:		Mon Mar 11 18:01:48 UTC 2024

 Modified Files:
 	src/sys/kern [netbsd-9]: sysv_shm.c

 Log Message:
 Pull up following revision(s) (requested by riastradh in ticket #1814):

 	sys/kern/sysv_shm.c: revision 1.142

 Avoid overflow when computing kern.ipc.shmmax. Keep shmmax (bytes) and
 shmall (pages) values aligned and use arithmetic everywhere instead
 of shifts.

 Should fix PR 57979


 To generate a diff of this commit:
 cvs rdiff -u -r1.135.2.4 -r1.135.2.5 src/sys/kern/sysv_shm.c

 Please note that diffs are not public domain; they are subject to the
 copyright notices on the relevant files.

From: "Martin Husemann" <martin@netbsd.org>
To: gnats-bugs@gnats.NetBSD.org
Cc: 
Subject: PR/57979 CVS commit: [netbsd-8] src/sys/kern
Date: Mon, 11 Mar 2024 18:04:54 +0000

 Module Name:	src
 Committed By:	martin
 Date:		Mon Mar 11 18:04:54 UTC 2024

 Modified Files:
 	src/sys/kern [netbsd-8]: sysv_shm.c

 Log Message:
 Pull up following revision(s) (requested by riastradh in ticket #1945):

 	sys/kern/sysv_shm.c: revision 1.142 (patch)

 Avoid overflow when computing kern.ipc.shmmax. Keep shmmax (bytes) and
 shmall (pages) values aligned and use arithmetic everywhere instead
 of shifts.

 Should fix PR 57979


 To generate a diff of this commit:
 cvs rdiff -u -r1.131.10.2 -r1.131.10.3 src/sys/kern/sysv_shm.c

 Please note that diffs are not public domain; they are subject to the
 copyright notices on the relevant files.

State-Changed-From-To: pending-pullups->closed
State-Changed-By: riastradh@NetBSD.org
State-Changed-When: Mon, 11 Mar 2024 22:24:29 +0000
State-Changed-Why:
fixed and pulled up


>Unformatted:

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