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Message-Id: <20201206063505.531798-1-morgan@kernel.org>
Date: Sat, 5 Dec 2020 22:35:05 -0800
From: "Andrew G. Morgan" <morgan@...nel.org>
To: akpm@...ux-foundation.org, linux-kernel@...r.kernel.org
Cc: "Andrew G. Morgan" <morgan@...nel.org>
Subject: [PATCH] Debug config option for including cpu thread id in fault and signal info
This option defaults to off.
I found this useful doing some fault isolation on an out of warranty
PC. I found one CPU core was consistently segfaulting and, together
with isolcpus=<n>,... I was able to breathe some life back into that
machine.
Since the si_errno value is generally hard-coded as zero for most signals,
userspace generally ignores it. Indeed, where I have tried it, the
userspace programs appear to be remarkably tolerant of this re-purposing
of the si_errno value.
Signed-off-by: Andrew G. Morgan <morgan@...nel.org>
---
init/Kconfig | 16 ++++++++++++++++
kernel/signal.c | 21 ++++++++++++++-------
2 files changed, 30 insertions(+), 7 deletions(-)
diff --git a/init/Kconfig b/init/Kconfig
index 02d13ae27abb..74f1202c449a 100644
--- a/init/Kconfig
+++ b/init/Kconfig
@@ -634,6 +634,22 @@ config PSI_DEFAULT_DISABLED
endmenu # "CPU/Task time and stats accounting"
+config SIG_ERRNO_HAS_THREAD_INFO
+ bool "Debug thread source for faults and interrupts"
+ default n
+ help
+ Select this to reveal to userspace, via siginfo_t.si_errno,
+ the HW thread number associated with a fault or signal
+ source. This feature has a number of HW debugging and
+ performance applications. For example, if a core <n> seems
+ to be unstable, isolcpus=<n> at boot can help avoid using
+ it.
+
+ The legacy and default value for this is mostly 0 for faults
+ and signals, see 'man sigaction' for details. To distinguish
+ thread 0 from this legacy, when the si_errno value holds a
+ valid thread number, its uppermost bit is also set to 1.
+
config CPU_ISOLATION
bool "CPU isolation"
depends on SMP || COMPILE_TEST
diff --git a/kernel/signal.c b/kernel/signal.c
index ef8f2a28d37c..523b93ec89f9 100644
--- a/kernel/signal.c
+++ b/kernel/signal.c
@@ -14,6 +14,7 @@
#include <linux/slab.h>
#include <linux/export.h>
#include <linux/init.h>
+#include <linux/sched.h>
#include <linux/sched/mm.h>
#include <linux/sched/user.h>
#include <linux/sched/debug.h>
@@ -1617,13 +1618,19 @@ send_sig(int sig, struct task_struct *p, int priv)
}
EXPORT_SYMBOL(send_sig);
+#ifdef CONFIG_SIG_ERRNO_HAS_THREAD_INFO
+#define ZERO_OR_THREAD_INDEX ((1<<(8*sizeof(int)-1))|task_cpu(current))
+#else
+#define ZERO_OR_THREAD_INDEX 0
+#endif
+
void force_sig(int sig)
{
struct kernel_siginfo info;
clear_siginfo(&info);
info.si_signo = sig;
- info.si_errno = 0;
+ info.si_errno = ZERO_OR_THREAD_INDEX;
info.si_code = SI_KERNEL;
info.si_pid = 0;
info.si_uid = 0;
@@ -1659,7 +1666,7 @@ int force_sig_fault_to_task(int sig, int code, void __user *addr
clear_siginfo(&info);
info.si_signo = sig;
- info.si_errno = 0;
+ info.si_errno = ZERO_OR_THREAD_INDEX;
info.si_code = code;
info.si_addr = addr;
#ifdef __ARCH_SI_TRAPNO
@@ -1691,7 +1698,7 @@ int send_sig_fault(int sig, int code, void __user *addr
clear_siginfo(&info);
info.si_signo = sig;
- info.si_errno = 0;
+ info.si_errno = ZERO_OR_THREAD_INDEX;
info.si_code = code;
info.si_addr = addr;
#ifdef __ARCH_SI_TRAPNO
@@ -1712,7 +1719,7 @@ int force_sig_mceerr(int code, void __user *addr, short lsb)
WARN_ON((code != BUS_MCEERR_AO) && (code != BUS_MCEERR_AR));
clear_siginfo(&info);
info.si_signo = SIGBUS;
- info.si_errno = 0;
+ info.si_errno = ZERO_OR_THREAD_INDEX;
info.si_code = code;
info.si_addr = addr;
info.si_addr_lsb = lsb;
@@ -1726,7 +1733,7 @@ int send_sig_mceerr(int code, void __user *addr, short lsb, struct task_struct *
WARN_ON((code != BUS_MCEERR_AO) && (code != BUS_MCEERR_AR));
clear_siginfo(&info);
info.si_signo = SIGBUS;
- info.si_errno = 0;
+ info.si_errno = ZERO_OR_THREAD_INDEX;
info.si_code = code;
info.si_addr = addr;
info.si_addr_lsb = lsb;
@@ -1740,7 +1747,7 @@ int force_sig_bnderr(void __user *addr, void __user *lower, void __user *upper)
clear_siginfo(&info);
info.si_signo = SIGSEGV;
- info.si_errno = 0;
+ info.si_errno = ZERO_OR_THREAD_INDEX;
info.si_code = SEGV_BNDERR;
info.si_addr = addr;
info.si_lower = lower;
@@ -1755,7 +1762,7 @@ int force_sig_pkuerr(void __user *addr, u32 pkey)
clear_siginfo(&info);
info.si_signo = SIGSEGV;
- info.si_errno = 0;
+ info.si_errno = ZERO_OR_THREAD_INDEX;
info.si_code = SEGV_PKUERR;
info.si_addr = addr;
info.si_pkey = pkey;
--
2.26.2
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