[PATCH 3/3] syscall: implement sendmsg+recvmsg in pure Perl

[Eric Wong <e@80x24.org>]

Socket::MsgHdr is only packaged for Debian and derivatives at
the moment, and Inline::C pulling in gcc/clang is a huge amount
of disk space and bandwidth for some users.

This enables disk space and/or bandwidth-limited users to use lei.

Only Linux guarantees a stable ABI and syscall numbers, but
that's the majority of our userbase.  FreeBSD users will still
have to use Inline::C (or get Socket::MsgHdr packaged).

x86, x32, and x86-64 are all currently supported, more to be added.
---
 devel/syscall-list         |  2 +
 lib/PublicInbox/Syscall.pm | 95 ++++++++++++++++++++++++++++++++++++++
 script/lei                 |  6 ++-
 t/cmd_ipc.t                | 13 +++++-
 4 files changed, 114 insertions(+), 2 deletions(-)

diff --git a/devel/syscall-list b/devel/syscall-list
index a6b1bfa7..d33a8a78 100755
--- a/devel/syscall-list
+++ b/devel/syscall-list
@@ -49,6 +49,8 @@ int main(void)
 	D(SYS_inotify_rm_watch);
 	D(SYS_prctl);
 	D(SYS_fstatfs);
+	D(SYS_sendmsg);
+	D(SYS_recvmsg);
 #ifdef FS_IOC_GETFLAGS
 	printf("FS_IOC_GETFLAGS=%#lx\nFS_IOC_SETFLAGS=%#lx\n",
 		(unsigned long)FS_IOC_GETFLAGS, (unsigned long)FS_IOC_SETFLAGS);
diff --git a/lib/PublicInbox/Syscall.pm b/lib/PublicInbox/Syscall.pm
index 806c192e..e9175ceb 100644
--- a/lib/PublicInbox/Syscall.pm
+++ b/lib/PublicInbox/Syscall.pm
@@ -2,6 +2,9 @@
 # specifically the Debian libsys-syscall-perl 0.25-6 version to
 # fix upstream regressions in 0.25.
 #
+# See devel/syscall-list in the public-inbox source tree for maintenance
+# <https://80x24.org/public-inbox.git>
+#
 # This license differs from the rest of public-inbox
 #
 # This module is Copyright (c) 2005 Six Apart, Ltd.
@@ -16,6 +19,7 @@ use strict;
 use v5.10.1;
 use parent qw(Exporter);
 use POSIX qw(ENOENT ENOSYS EINVAL O_NONBLOCK);
+use Socket qw(SOL_SOCKET SCM_RIGHTS);
 use Config;
 
 # $VERSION = '0.25'; # Sys::Syscall version
@@ -42,8 +46,19 @@ use constant {
 	EPOLL_CTL_ADD => 1,
 	EPOLL_CTL_DEL => 2,
 	EPOLL_CTL_MOD => 3,
+	SIZEOF_int => $Config{intsize},
+	SIZEOF_size_t => $Config{sizesize},
+	NUL => "\0",
+};
+
+use constant {
+	TMPL_size_t => SIZEOF_size_t == 8 ? 'Q' : 'L',
+	BYTES_4_hole => SIZEOF_size_t == 8 ? 'L' : '',
+	# cmsg_len, cmsg_level, cmsg_type
+	SIZEOF_cmsghdr => SIZEOF_int * 2 + SIZEOF_size_t,
 };
 
+my @BYTES_4_hole = BYTES_4_hole ? (0) : ();
 our $loaded_syscall = 0;
 
 sub _load_syscall {
@@ -68,6 +83,7 @@ our (
      $SYS_renameat2,
      );
 
+my ($SYS_sendmsg, $SYS_recvmsg);
 my $SYS_fstatfs; # don't need fstatfs64, just statfs.f_type
 my ($FS_IOC_GETFLAGS, $FS_IOC_SETFLAGS);
 my $SFD_CLOEXEC = 02000000; # Perl does not expose O_CLOEXEC
@@ -99,6 +115,8 @@ if ($^O eq "linux") {
         $SYS_signalfd4 = 327;
         $SYS_renameat2 //= 353;
 	$SYS_fstatfs = 100;
+	$SYS_sendmsg = 370;
+	$SYS_recvmsg = 372;
 	$FS_IOC_GETFLAGS = 0x80046601;
 	$FS_IOC_SETFLAGS = 0x40046602;
     } elsif ($machine eq "x86_64") {
@@ -108,6 +126,8 @@ if ($^O eq "linux") {
         $SYS_signalfd4 = 289;
 	$SYS_renameat2 //= 316;
 	$SYS_fstatfs = 138;
+	$SYS_sendmsg = 46;
+	$SYS_recvmsg = 47;
 	$FS_IOC_GETFLAGS = 0x80086601;
 	$FS_IOC_SETFLAGS = 0x40086602;
     } elsif ($machine eq 'x32') {
@@ -117,6 +137,8 @@ if ($^O eq "linux") {
         $SYS_signalfd4 = 1073742113;
 	$SYS_renameat2 //= 0x40000000 + 316;
 	$SYS_fstatfs = 138;
+	$SYS_sendmsg = 0x40000206;
+	$SYS_recvmsg = 0x40000207;
 	$FS_IOC_GETFLAGS = 0x80046601;
 	$FS_IOC_SETFLAGS = 0x40046602;
     } elsif ($machine eq 'sparc64') {
@@ -378,6 +400,79 @@ sub nodatacow_dir {
 	if (open my $fh, '<', $_[0]) { nodatacow_fh($fh) }
 }
 
+sub CMSG_ALIGN ($) { ($_[0] + SIZEOF_size_t - 1) & ~(SIZEOF_size_t - 1) }
+use constant CMSG_ALIGN_SIZEOF_cmsghdr => CMSG_ALIGN(SIZEOF_cmsghdr);
+sub CMSG_SPACE ($) { CMSG_ALIGN($_[0]) + CMSG_ALIGN_SIZEOF_cmsghdr }
+sub CMSG_LEN ($) { CMSG_ALIGN_SIZEOF_cmsghdr + $_[0] }
+
+if (defined($SYS_sendmsg) && defined($SYS_recvmsg)) {
+no warnings 'once';
+*send_cmd4 = sub ($$$$) {
+	my ($sock, $fds, undef, $flags) = @_;
+	my $iov = pack('P'.TMPL_size_t,
+			$_[2] // NUL, length($_[2] // NUL) || 1);
+	my $cmsghdr = pack(TMPL_size_t . # cmsg_len
+			'LL' .  # cmsg_level, cmsg_type,
+			('i' x scalar(@$fds)),
+			CMSG_LEN(scalar(@$fds) * SIZEOF_int), # cmsg_len
+			SOL_SOCKET, SCM_RIGHTS, # cmsg_{level,type}
+			@$fds); # CMSG_DATA
+	my $mh = pack('PL' . # msg_name, msg_namelen (socklen_t (U32))
+			BYTES_4_hole . # 4-byte padding on 64-bit
+			'P'.TMPL_size_t . # msg_iov, msg_iovlen,
+			'P'.TMPL_size_t . # msg_control, msg_controllen,
+			'i', # msg_flags
+			NUL, 0, # msg_name, msg_namelen (unused)
+			@BYTES_4_hole,
+			$iov, 1, # msg_iov, msg_iovlen
+			$cmsghdr, # msg_control
+			CMSG_SPACE(scalar(@$fds) * SIZEOF_int), # msg_controllen
+			0); # msg_flags
+	my $sent;
+	my $try = 0;
+	do {
+		$sent = syscall($SYS_sendmsg, fileno($sock), $mh, $flags);
+	} while ($sent < 0 &&
+			($!{ENOBUFS} || $!{ENOMEM} || $!{ETOOMANYREFS}) &&
+			(++$try < 50) &&
+			warn "sleeping on sendmsg: $! (#$try)\n" &&
+			select(undef, undef, undef, 0.1) == 0);
+	$sent >= 0 ? $sent : undef;
+};
+
+*recv_cmd4 = sub ($$$) {
+	my ($sock, undef, $len) = @_;
+	vec($_[1], ($len + 1) * 8, 1) = 0;
+	vec(my $cmsghdr = '', 256 * 8 - 1, 1) = 1;
+	my $iov = pack('P'.TMPL_size_t, $_[1], $len);
+	my $mh = pack('PL' . # msg_name, msg_namelen (socklen_t (U32))
+			BYTES_4_hole . # 4-byte padding on 64-bit
+			'P'.TMPL_size_t . # msg_iov, msg_iovlen,
+			'P'.TMPL_size_t . # msg_control, msg_controllen,
+			'i', # msg_flags
+			NUL, 0, # msg_name, msg_namelen (unused)
+			@BYTES_4_hole,
+			$iov, 1, # msg_iov, msg_iovlen
+			$cmsghdr, # msg_control
+			256, # msg_controllen
+			0); # msg_flags
+	my $r = syscall($SYS_recvmsg, fileno($sock), $mh, 0);
+	return (undef) if $r < 0; # $! set
+	substr($_[1], $r, length($_[1]), '');
+	my @ret;
+	if ($r > 0) {
+		my ($len, $lvl, $type, @fds) = unpack(TMPL_size_t . # cmsg_len
+					'LLi*', # cmsg_level, cmsg_type, @fds
+					$cmsghdr);
+		if ($lvl == SOL_SOCKET && $type == SCM_RIGHTS) {
+			$len -= CMSG_ALIGN_SIZEOF_cmsghdr;
+			@ret = @fds[0..(($len / SIZEOF_int) - 1)];
+		}
+	}
+	@ret;
+};
+}
+
 1;
 
 =head1 WARRANTY
diff --git a/script/lei b/script/lei
index 5cad19d7..adef9944 100755
--- a/script/lei
+++ b/script/lei
@@ -1,5 +1,5 @@
 #!perl -w
-# Copyright (C) 2020-2021 all contributors <meta@public-inbox.org>
+# Copyright (C) all contributors <meta@public-inbox.org>
 # License: AGPL-3.0+ <https://www.gnu.org/licenses/agpl-3.0.txt>
 use strict;
 use v5.10.1;
@@ -9,6 +9,10 @@ my $narg = 5;
 my $sock;
 my $recv_cmd = PublicInbox::CmdIPC4->can('recv_cmd4');
 my $send_cmd = PublicInbox::CmdIPC4->can('send_cmd4') // do {
+	require PublicInbox::Syscall;
+	$recv_cmd = PublicInbox::Syscall->can('recv_cmd4');
+	PublicInbox::Syscall->can('send_cmd4');
+} // do {
 	my $inline_dir = $ENV{PERL_INLINE_DIRECTORY} //= (
 			$ENV{XDG_CACHE_HOME} //
 			( ($ENV{HOME} // '/nonexistent').'/.cache' )
diff --git a/t/cmd_ipc.t b/t/cmd_ipc.t
index dd90fa2a..75697a15 100644
--- a/t/cmd_ipc.t
+++ b/t/cmd_ipc.t
@@ -1,5 +1,5 @@
 #!perl -w
-# Copyright (C) 2021 all contributors <meta@public-inbox.org>
+# Copyright (C) all contributors <meta@public-inbox.org>
 # License: AGPL-3.0+ <https://www.gnu.org/licenses/agpl-3.0.txt>
 use strict;
 use v5.10.1;
@@ -142,4 +142,15 @@ SKIP: {
 	}
 }
 
+SKIP: {
+	skip 'not Linux', 1 if $^O ne 'linux';
+	require_ok 'PublicInbox::Syscall';
+	$send = PublicInbox::Syscall->can('send_cmd4') or
+		skip 'send_cmd4 not defined for arch';
+	$recv = PublicInbox::Syscall->can('recv_cmd4') or
+		skip 'recv_cmd4 not defined for arch';
+	$do_test->(SOCK_STREAM, 0, 'PP Linux stream');
+	$do_test->($SOCK_SEQPACKET, MSG_EOR, 'PP Linux seqpacket');
+}
+
 done_testing;