Insufficient sigaltstack size used by CPython prevents extensions from using new ISA

[oleksandr-pavlyk]

BPO	46968
Nosy	@vstinner, @ambv, @pablogsal, @miss-islington, @oleksandr-pavlyk
PRs	bpo-46968: Query altstack size dynamically on Linux kernel >=5.14 #31789 bpo-46968: Add os.sysconf_names['SC_MINSIGSTKSZ'] #31824 [3.10] bpo-46968: Fix faulthandler for Sapphire Rapids Xeon (GH-31789) #31830 [3.9] bpo-46968: Fix faulthandler for Sapphire Rapids Xeon (GH-31789) #31831 bpo-46968: Check for 'sys/auxv.h' in the configure script #31961 [3.10] bpo-46968: Check for 'sys/auxv.h' in the configure script (GH-1961) #31974 [3.9] bpo-46968: Check for 'sys/auxv.h' in the configure script (GH-31961) #31975

^{Note: these values reflect the state of the issue at the time it was migrated and might not reflect the current state.}

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GitHub fields:

assignee = None
closed_at = <Date 2022-03-18.13:33:30.380>
created_at = <Date 2022-03-09.19:07:02.730>
labels = ['extension-modules', 'type-bug', '3.9', '3.10', '3.11']
title = 'Insufficient sigaltstack size used by CPython prevents extensions from using new ISA'
updated_at = <Date 2022-03-18.14:15:32.541>
user = 'https://github.com/oleksandr-pavlyk'

bugs.python.org fields:

activity = <Date 2022-03-18.14:15:32.541>
actor = 'vstinner'
assignee = 'none'
closed = True
closed_date = <Date 2022-03-18.13:33:30.380>
closer = 'pablogsal'
components = ['Extension Modules']
creation = <Date 2022-03-09.19:07:02.730>
creator = 'oleksandr-pavlyk'
dependencies = []
files = []
hgrepos = []
issue_num = 46968
keywords = ['patch']
message_count = 21.0
messages = ['414809', '414938', '414939', '414941', '414943', '414949', '414950', '414957', '414960', '414965', '415402', '415403', '415404', '415405', '415406', '415407', '415483', '415487', '415496', '415497', '415500']
nosy_count = 6.0
nosy_names = ['vstinner', 'lukasz.langa', 'lkollar', 'pablogsal', 'miss-islington', 'oleksandr-pavlyk']
pr_nums = ['31789', '31824', '31830', '31831', '31961', '31974', '31975']
priority = None
resolution = 'fixed'
stage = 'resolved'
status = 'closed'
superseder = None
type = 'behavior'
url = 'https://bugs.python.org/issue46968'
versions = ['Python 3.9', 'Python 3.10', 'Python 3.11']

[oleksandr-pavlyk]

The following snippet illustrates request by an extension to use AMX registers:

// no increase sigaltstack size fix will not wark till we fix python
void enable_amx_no_fix()
{

    unsigned long bitmask;
    long rc;

    rc = syscall(SYS_arch_prctl, ARCH_REQ_XCOMP_PERM, XFEATURE_XTILEDATA);
    if (rc) {
        printf("The kernel rejects the AMX use.\n");
        printf("errno %d\n",errno);
    } else {
        printf("The kernel allows to use AMX.\n");
    }


    rc = syscall(SYS_arch_prctl, ARCH_GET_XCOMP_PERM, &bitmask);

    if (rc) {
        printf("rc error\n");
    } else  {
        if (( bitmask & XFEATURE_MASK_XTILEDATA) == 0){
            printf("verify AMX permission faild bitmask %ld\n",bitmask);
        }

    }
}

This request fails on the account of too small a size for sigaltstack used by CPython allocated in Modules/faulthandler.c

The stack size used is 2*SIGSTKSZ, and does not take hardware capabilities into account.

Linux kernel 5.14 adds support to query minimum size of sigaltstack dynamically via getauxval(AT_MINSIGSTKSZ).

AMX support is added in Linux kernel 5.16

CPython should make use of this when built against more recent Linux kernels.