Date: Sun, 11 Jan 2009 00:35:13 GMT From: Gary Byers <gb@clozure.com> To: freebsd-gnats-submit@FreeBSD.org Subject: amd64/130355: i386_set_fsbase() doesn't seem to set %fs for 32-bit process under 7.1/amd64 Message-ID: <200901110035.n0B0ZDX5089905@www.freebsd.org> Resent-Message-ID: <200901110040.n0B0e55Q046291@freefall.freebsd.org>
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>Number: 130355 >Category: amd64 >Synopsis: i386_set_fsbase() doesn't seem to set %fs for 32-bit process under 7.1/amd64 >Confidential: no >Severity: critical >Priority: high >Responsible: freebsd-amd64 >State: open >Quarter: >Keywords: >Date-Required: >Class: sw-bug >Submitter-Id: current-users >Arrival-Date: Sun Jan 11 00:40:05 UTC 2009 >Closed-Date: >Last-Modified: >Originator: Gary Byers >Release: 7.1-RELEASE/amd64 >Organization: Clozure Associates >Environment: FreeBSD f71.abq.clozure.com 7.1-RELEASE FreeBSD 7.1-RELEASE #0: Thu Jan 1 08:58:24 UTC 2009 root@driscoll.cse.buffalo.edu:/usr/obj/usr/src/sys/GENERIC amd64 >Description: i386_set_fsbase(void *addr) has the effect of causing memory references relative to %fs to reference the linear address "addr"; part of this effect is achieved by loading a segment descriptor (GSEL(GUFS_SEL, SEL_UPL), which is generally = 0x13) into %fs on return from the sysarch syscall. This works as expected on (at least) i386 versions of 6.4-RELEASE, 7.0-RELEASE, and 7.1-RELEASE and on amd64 versions of 6.4-RELEASE and 7.0-RELEASE, but on 7.1-RELEASE/amd64 the i386_set_fsbase() call returns 0 but %fs is unchanged on return and subsequent attempts to reference memory relative to %fs seem to be equivalent to references relative to address 0. >How-To-Repeat: Compile (in a 32-bit FreeBSD/i386 environment) a small program that uses i386_set_fsbase() and checks to ensure that %fs has changed on successful return from the call to i386_set_fsbase(). Run the program on i386 versions of FreeBSD (6.4, 7.0, 7.1) and on amd64 versions of 6.4 and 7.0 and note that %fs is changed by the syscall. Run the program on 7.1-RELEASE/amd64 and note that it fails. (I'm not really sure if I'm supposed to provide a simple test case on the web form that I'm using to report this; I can easily do so if requested.) >Fix: Unknown. To the extent that I understand the issue, it seems to have to do with code that tries to ensure that the correct value is loaded into %fs on return from the syscall and on subsequent context switches; the code that actually sets the fsbase msr seems to be unchanged between the released versions of 7.0 and 7.1. >Release-Note: >Audit-Trail: >Unformatted:
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