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bpo-46968: Query altstack size dynamically on Linux kernel >=5.14 #31789

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Merged
merged 3 commits into from
Mar 11, 2022
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bpo-46968: Query altstack size dynamically on Linux kernel >=5.14 #31789

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3a32f59
Added check for linux/auxvec.h in configure
oleksandr-pavlyk Mar 10, 2022
29f783e
Query altstack size dynamically on Linux kernel >=5.14
oleksandr-pavlyk Mar 8, 2022
2a2cbd6
📜🤖 Added by blurb_it.
blurb-it[bot] Mar 10, 2022
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5 changes: 5 additions & 0 deletions Misc/NEWS.d/next/Library/2022-03-10-14-51-11.bpo-46968.ym2QxL.rst
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@@ -0,0 +1,5 @@
:mod:`faulthandler`: On Linux 5.14 and newer, dynamically determine size of
signal handler stack size CPython allocates using ``getauxval(AT_MINSIGSTKSZ)``.
This changes allows for Python extension's request to Linux kernel to use
AMX_TILE instruction set on Sapphire Rapids Xeon processor to succeed,
unblocking use of the ISA in frameworks.
26 changes: 20 additions & 6 deletions Modules/faulthandler.c
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Expand Up @@ -20,6 +20,17 @@
# include <sys/resource.h>
#endif

/* Using an alternative stack requires sigaltstack()
and sigaction() SA_ONSTACK */
#if defined(HAVE_SIGALTSTACK) && defined(HAVE_SIGACTION)
# define FAULTHANDLER_USE_ALT_STACK
#endif

#if defined(FAULTHANDLER_USE_ALT_STACK) && defined(HAVE_LINUX_AUXVEC_H)
# include <linux/auxvec.h>
# include <sys/auxv.h>
#endif

/* Allocate at maximum 100 MiB of the stack to raise the stack overflow */
#define STACK_OVERFLOW_MAX_SIZE (100 * 1024 * 1024)

Expand Down Expand Up @@ -137,12 +148,6 @@ static fault_handler_t faulthandler_handlers[] = {
static const size_t faulthandler_nsignals = \
Py_ARRAY_LENGTH(faulthandler_handlers);

/* Using an alternative stack requires sigaltstack()
and sigaction() SA_ONSTACK */
#if defined(HAVE_SIGALTSTACK) && defined(HAVE_SIGACTION)
# define FAULTHANDLER_USE_ALT_STACK
#endif

#ifdef FAULTHANDLER_USE_ALT_STACK
static stack_t stack;
static stack_t old_stack;
Expand Down Expand Up @@ -1373,6 +1378,15 @@ _PyFaulthandler_Init(int enable)
signal handler uses more than SIGSTKSZ bytes of stack memory on some
platforms. */
stack.ss_size = SIGSTKSZ * 2;
#ifdef AT_MINSIGSTKSZ
/* bpo-46968: Query Linux for minimal stack size to ensure signal delivery
for the hardware running CPython. This OS feature is available in
Linux kernel version >= 5.14 */
unsigned long at_minstack_size = getauxval(AT_MINSIGSTKSZ);
if (at_minstack_size != 0) {
stack.ss_size = SIGSTKSZ + at_minstack_size;
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I'm surprised that AT_MINSIGSTKSZ doesn't return directly the value which should be passed to sigaltstack(), but it seems like your code is the correct way to use the value.

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The AT_MINSIGSTKSZ is a preprocessor variable. The getauxval checks hardware capabilities (e.g. total size of CPU state to be saved at context switches) and may return different values on different platforms.

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I mean that I expected to write (simplified code without error handling): stack.ss_size = getauxval(AT_MINSIGSTKSZ), rather than stack.ss_size = SIGSTKSZ + getauxval(AT_MINSIGSTKSZ). But your code looks correct according to the doc that I found on the Internet. Like the official Linux documentation:

ss.ss_size = getauxval(AT_MINSIGSTKSZ) + SIGSTKSZ;

ref: https://docs.kernel.org/x86/elf_auxvec.html#introduction

}
#endif
#endif

memset(&thread, 0, sizeof(thread));
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2 changes: 1 addition & 1 deletion configure
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2 changes: 1 addition & 1 deletion configure.ac
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Expand Up @@ -2375,7 +2375,7 @@ AC_DEFINE(STDC_HEADERS, 1, [Define to 1 if you have the ANSI C header files.])
# checks for header files
AC_CHECK_HEADERS([ \
alloca.h asm/types.h bluetooth.h conio.h crypt.h direct.h dlfcn.h endian.h errno.h fcntl.h grp.h \
ieeefp.h io.h langinfo.h libintl.h libutil.h linux/memfd.h linux/random.h linux/soundcard.h \
ieeefp.h io.h langinfo.h libintl.h libutil.h linux/auxvec.h linux/memfd.h linux/random.h linux/soundcard.h \
linux/tipc.h linux/wait.h netinet/in.h netpacket/packet.h poll.h process.h pthread.h pty.h \
sched.h setjmp.h shadow.h signal.h spawn.h stropts.h sys/audioio.h sys/bsdtty.h sys/devpoll.h \
sys/endian.h sys/epoll.h sys/event.h sys/eventfd.h sys/file.h sys/ioctl.h sys/kern_control.h \
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3 changes: 3 additions & 0 deletions pyconfig.h.in
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Expand Up @@ -661,6 +661,9 @@
/* Define to 1 if you have the `linkat' function. */
#undef HAVE_LINKAT

/* Define to 1 if you have the <linux/auxvec.h> header file. */
#undef HAVE_LINUX_AUXVEC_H

/* Define to 1 if you have the <linux/can/bcm.h> header file. */
#undef HAVE_LINUX_CAN_BCM_H

Expand Down