LLVM code generation can attempt to load from a variable in the next condition of an expression under certain circumstances, thus turning the following condition:
if ( system_state < SYS_STATE_active && opt_bootscrub == BOOTSCRUB_IDLE ) Into: 0xffff82d080223967 <+103>: cmpl $0x3,0x37b032(%rip) # 0xffff82d08059e9a0 <system_state> 0xffff82d08022396e <+110>: setb -0x29(%rbp) 0xffff82d080223972 <+114>: cmpl $0x2,0x228a8b(%rip) # 0xffff82d08044c404 <opt_bootscrub> Such code will trigger a page fault if system_state >= SYS_STATE_active because opt_bootscrub will be unmapped. Fix this by making opt_bootscrub non-init, thus preventing the page fault. The LLVM bug with the discussion about this issue can be found at: https://bugs.llvm.org/show_bug.cgi?id=39707 I haven't been able to find any other instances of such conditional expression that uses system_state together with an init variable or function. Signed-off-by: Roger Pau Monné <roger....@citrix.com> Reviewed-by: Sergey Dyasli <sergey.dya...@citrix.com> Acked-by: Andrew Cooper <andrew.coop...@citrix.com> Acked-by: Julien Grall <julien.gr...@arm.com> Acked-by: Wei Liu <wei.l...@citrix.com> --- Cc: Andrew Cooper <andrew.coop...@citrix.com> Cc: George Dunlap <george.dun...@eu.citrix.com> Cc: Ian Jackson <ian.jack...@eu.citrix.com> Cc: Jan Beulich <jbeul...@suse.com> Cc: Julien Grall <julien.gr...@arm.com> Cc: Konrad Rzeszutek Wilk <konrad.w...@oracle.com> Cc: Stefano Stabellini <sstabell...@kernel.org> Cc: Tim Deegan <t...@xen.org> Cc: Wei Liu <wei.l...@citrix.com> Cc: Sergey Dyasli <sergey.dya...@citrix.com> --- Changes since v1: - Make opt_bootscrub read mostly. - Add a comment about why opt_bootscrub is not in the init section. --- xen/common/page_alloc.c | 9 ++++++++- 1 file changed, 8 insertions(+), 1 deletion(-) diff --git a/xen/common/page_alloc.c b/xen/common/page_alloc.c index 08ee8cfbb9..b4086781c4 100644 --- a/xen/common/page_alloc.c +++ b/xen/common/page_alloc.c @@ -166,7 +166,14 @@ enum bootscrub_mode { BOOTSCRUB_ON, BOOTSCRUB_IDLE, }; -static enum bootscrub_mode __initdata opt_bootscrub = BOOTSCRUB_IDLE; +/* + * opt_bootscrub should live in the init section, since it's not accessed + * afterwards. However at least LLVM assumes there are no side effects of + * accessing the variable, and optimizes the condition so opt_bootscrub is + * read regardless of the value of system_state: + * https://bugs.llvm.org/show_bug.cgi?id=39707 + */ +static enum bootscrub_mode __read_mostly opt_bootscrub = BOOTSCRUB_IDLE; static int __init parse_bootscrub_param(const char *s) { /* Interpret 'bootscrub' alone in its positive boolean form */ -- 2.19.1 _______________________________________________ Xen-devel mailing list Xen-devel@lists.xenproject.org https://lists.xenproject.org/mailman/listinfo/xen-devel
