lists.openwall.net | lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC | |
Open Source and information security mailing list archives
| ||
|
Date: Fri, 21 Aug 2015 07:50:05 -0400 From: Prarit Bhargava <prarit@...hat.com> To: Ingo Molnar <mingo@...nel.org> CC: linux-kernel@...r.kernel.org, Thomas Gleixner <tglx@...utronix.de>, Ingo Molnar <mingo@...hat.com>, "H. Peter Anvin" <hpa@...or.com>, x86@...nel.org, Linus Torvalds <torvalds@...ux-foundation.org>, Andrew Morton <akpm@...ux-foundation.org>, Peter Zijlstra <a.p.zijlstra@...llo.nl> Subject: Re: [PATCH] x86, bitops, variable_test_bit should return 1 not -1 on a match On 08/21/2015 02:51 AM, Ingo Molnar wrote: > > * Prarit Bhargava <prarit@...hat.com> wrote: > >> This issue was noticed while debugging a CPU hotplug issue. On x86 >> with (NR_CPUS > 1) the cpu_online() define is cpumask_test_cpu(). >> cpumask_test_cpu() should return 1 if the cpu is set in cpumask and >> 0 otherwise. >> >> However, cpumask_test_cpu() returns -1 if the cpu in the cpumask is >> set and 0 otherwise. This happens because cpumask_test_cpu() calls >> test_bit() which is a define that will call variable_test_bit(). >> >> variable_test_bit() calls the assembler instruction sbb (Subtract >> with Borrow, " Subtracts the source from the destination, and subtracts 1 >> extra if the Carry Flag is set. Results are returned in "dest".) >> >> A bit match results in -1 being returned from variable_test_bit() if a >> match occurs, not 1 as the function is supposed to. This can be easily >> resolved by adding a "!!" to force 0 or 1 as a return. >> >> It looks like the code never does, for example, (test_bit() == 1) so this >> change should not have any impact. >> >> Cc: Thomas Gleixner <tglx@...utronix.de> >> Cc: Ingo Molnar <mingo@...hat.com> >> Cc: "H. Peter Anvin" <hpa@...or.com> >> Cc: x86@...nel.org >> Cc: linux-kernel@...r.kernel.org >> Signed-off-by: Prarit Bhargava <prarit@...hat.com> >> --- >> arch/x86/include/asm/bitops.h | 2 +- >> 1 file changed, 1 insertion(+), 1 deletion(-) >> >> diff --git a/arch/x86/include/asm/bitops.h b/arch/x86/include/asm/bitops.h >> index cfe3b95..a87a5fb 100644 >> --- a/arch/x86/include/asm/bitops.h >> +++ b/arch/x86/include/asm/bitops.h >> @@ -320,7 +320,7 @@ static inline int variable_test_bit(long nr, volatile const unsigned long *addr) >> : "=r" (oldbit) >> : "m" (*(unsigned long *)addr), "Ir" (nr)); >> >> - return oldbit; >> + return !!oldbit; >> } >> >> #if 0 /* Fool kernel-doc since it doesn't do macros yet */ > > Ok, I think this is a good fix to improve the robustness of this primitive, unless > someone objects. > > I tried to find the CPU hotplug code that broke with cpu_online() returning -1 but > failed - all current mainline usage sites seem to be testing for nonzero in one > way or another. Could you please point it out? I'm sorry Ingo, I think my description may have confused you. I was debugging a cpu hotplug issue[1] and did printk("cpu %d cpu online status %d\n", cpu, cpu_online(cpu)); as a debug printk. This printed out cpu 3 cpu online status -1 which was really confusing. That lead me down the rabbit hole of looking at the sbb assembler instruction in variable_test_bit() to figure out why I was seeing -1. P. [1] The bug looks like it has to do with the system's firmware, not cpu hotplug. -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@...r.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
Powered by blists - more mailing lists