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

@vstinner

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']

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.

The configure file is checking for "linux/auxvec.h"

checking linux/auxvec.h usability... yes

but the code is including "sys/auxvec.h"

The code is assuming that if linux/auxvec.h then sys/auxv.h will be available, which is wrong.

This may be quite bad, because this means that 3.10 and 3.9 doesn't build in CentOS 6, which is used for manylinux2010 wheels

Commit 393e2bf has broken CPython in RedHat 6

Too bad that we want to support RHEL 6 but have no buildbot for that.

New changeset 8e3fde7 by Pablo Galindo Salgado in branch 'main':
bpo-46968: Check for 'sys/auxv.h' in the configure script (GH-31961)
8e3fde7

New changeset a12ef81 by Pablo Galindo Salgado in branch '3.9':
[3.9] bpo-46968: Check for 'sys/auxv.h' in the configure script (GH-31961). (GH-31975)
a12ef81

New changeset 6fd9737 by Pablo Galindo Salgado in branch '3.10':
[3.10] bpo-46968: Check for 'sys/auxv.h' in the configure script (GH-31961). (GH-31974)
6fd9737

Thanks for the fix Pablo.

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Insufficient sigaltstack size used by CPython prevents extensions from using new ISA #91124

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Insufficient sigaltstack size used by CPython prevents extensions from using new ISA #91124

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oleksandr-pavlyk commented on Mar 9, 2022

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pablogsal commented on Mar 17, 2022

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vstinner commented on Mar 18, 2022

miss-islington commented on Mar 18, 2022

pablogsal commented on Mar 18, 2022

pablogsal commented on Mar 18, 2022

vstinner commented on Mar 18, 2022

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