最近写 Perl 程序时遇到一个很奇怪的问题：
Insecure dependency in unlink while running with -T switch at ../tmpfile.pl line 44.
经过检查，发现这是 Perl 语言一个特性，在运行时使用 -w 或 -T 都意味着 “万无一失” 标志。
-T 标志意味着任何来自外部世界的值（例如从文件读取）都被认为是潜在的威胁，并且不允许在与系统相关的操作中使用这些值，比如写文件、执行系统命令等等。
-w 作用与 use warning 相同，会抛出一些有用的警告信息，如 using uninitialized variable。
为了更清晰的表述该问题，我抽象出一个简单的示例程序：
#!/usr/bin/perl -wT
use strict;
use warnings;
use Digest::MD5;
my $DIR_PATH="/var/tmp";
my $PREFIX = "somedemotmpfile";
sub make_file {
my ($filename) = @_;
open my $fh, '>', $filename;
print {$fh} "1" . "\n";
print {$fh} "2" . "\n";
print {$fh} "3" . "\n";
close $fh;
}
sub make_tmpfile {
foreach (1..5){
my $tmpfilename = "$DIR_PATH/$PREFIX-" . Digest::MD5::md5_hex($_ . time() . $$);
make_file($tmpfilename);
}
}
sub clean_tmpfile {
opendir (my $dh, $DIR_PATH) || die "Can not open $DIR_PATH/n";
my @dots=grep { !/^\.+$/ } readdir($dh);
closedir($dh);
foreach my $file (@dots)
{
my $afile = "$DIR_PATH/$file";
my $now = time();
if (-e $afile && $afile =~ m/(^.*$PREFIX.*$)/) {
#$afile = $1;
my $mtime = (stat ($afile))[9];
my $margin = $now - $mtime;
print("$afile - Last change: $mtime - now: $now - margin(s): $margin\n");
eval {
unlink $afile;
};
warn $@ if $@;
}
}
}
sub main {
make_tmpfile();
clean_tmpfile();
}
main();
执行该程序，得到如下输出：
# perl -T ../tmpfile.pl
/var/tmp/somedemotmpfile-e48d74ec998a1462661eb11b7576d7e5 - Last change: 1658904122 - now: 1658904122 - margin(s): 0
Insecure dependency in unlink while running with -T switch at ../tmpfile.pl line 44.
/var/tmp/somedemotmpfile-a071ba8e02d34ef2878d7a698a22b93c - Last change: 1658904122 - now: 1658904122 - margin(s): 0
Insecure dependency in unlink while running with -T switch at ../tmpfile.pl line 44.
/var/tmp/somedemotmpfile-1a63c4b7965dc50c519e7aa68c8b081a - Last change: 1658904122 - now: 1658904122 - margin(s): 0
Insecure dependency in unlink while running with -T switch at ../tmpfile.pl line 44.
可以看到，当我从文件系统读取一些文件，并尝试直接删除这些问题时，这步操作被阻止，并报出警告 Insecure dependency in unlink while running with -T switch。
为了消除“污染”，最简单的方法是使用严格正则匹配后的结果再做操作，代码修改如下：
diff --git a/study_perl/tmpfile.pl b/study_perl/tmpfile.pl
index 6520a25..51ef684 100644
--- a/study_perl/tmpfile.pl
+++ b/study_perl/tmpfile.pl
@@ -36,7 +36,7 @@ sub clean_tmpfile {
my $afile = "$DIR_PATH/$file";
my $now = time();
if (-e $afile && $afile =~ m/(^.*$PREFIX.*$)/) {
- #$afile = $1;
+ $afile = $1;
my $mtime = (stat ($afile))[9];
my $margin = $now - $mtime;
print("$afile - Last change: $mtime - now: $now - margin(s): $margin\n");
再次尝试运行，得到正确的结果：
# perl -T ../tmpfile.pl
/var/tmp/somedemotmpfile-f036c279daa16297818f6ec2dad9f338 - Last change: 1658904375 - now: 1658904375 - margin(s): 0
/var/tmp/somedemotmpfile-f532d49f86ae1c486ec593c71a073e73 - Last change: 1658904375 - now: 1658904375 - margin(s): 0
/var/tmp/somedemotmpfile-e48d74ec998a1462661eb11b7576d7e5 - Last change: 1658904122 - now: 1658904375 - margin(s): 253
/var/tmp/somedemotmpfile-a071ba8e02d34ef2878d7a698a22b93c - Last change: 1658904122 - now: 1658904375 - margin(s): 253
/var/tmp/somedemotmpfile-a140c4f6095a0431194a91ead41ce605 - Last change: 1658904375 - now: 1658904375 - margin(s): 0
/var/tmp/somedemotmpfile-1a63c4b7965dc50c519e7aa68c8b081a - Last change: 1658904122 - now: 1658904375 - margin(s): 253
执行成功，且删除了之前的残留文件。
经过这次问题解决，发现 Perl 在安全方面的特性值得学习，在编译或解释层面阻挡常见安全操作被执行，可以使得我们写出更加安全的代码。
即使不写 perl 代码，使用其他语言写程序时也可有所启发。