build/*.m4: Remove Bashisms/POSIX-ify the M4 helper macros

[BalkanMadman]

This commit makes it possible to build ModSecurity on systems where /bin/sh is a POSIX-compliant shell that is not Bash. Debian, Alpine Linux, and Gentoo Linux with the system shell set to not Bash, are examples of such systems.

Previously, the helper macros contained two types of Bashisms:

• The == comparison operator. Very easy to change, as the proper POSIX-compliant form is =. For example:

if test "${var}" == "myvalue"  ->  if test "${var}" = "myvalue"

• The '-a' (and) operator in the 'test' builtin. The '-a' and '-o' operators were removed in POSIX 2024 (Issue 8). The correct form is to use the '&&' and '||' operators respectively. For instance:

if test -d "${var}" -a -r "${var}/file"  ->  if test -d "${var}" && test -r "${var}/file"

Bug: https://bugs.gentoo.org/887135

what

• The Bash == operator changed to the POSIX-compliant =.
• The obsolete and removed in POSIX 2024 -a operator changed to the compliant &&.

why

• Unable to build ModSecurity on systems where /bin/sh is not Bash (is dash, ash, etc.). See the Gentoo bug below.

references

• The Gentoo bug: https://bugs.gentoo.org/887135

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud

[airween]

Please take a look at this comment from @theseion. I think he is right, and (unfortunately) we should review all complex conditions.

Moreover, I don't know: do we want to handle the yes value in case of all --with... options?

Here are the output of different scenarios (tested with these expressions):

if test "x${with_pcre}" = "x" && test "x${with_pcre}" != "xno"; then echo "TRUE"; else echo "FALSE";
if test "x${with_pcre}" = "x" || test "x${with_pcre}" != "xno"; then echo "TRUE"; else echo "FALSE";

Value of with_pcre	expected output	real output	meanings
condition: &&
"" (empty)	TRUE	TRUE	--with-pcre - we want to use PCRE
yes	TRUE	FALSE!	--with-pcre=yes - we want to use PCRE
no	FALSE	FALSE	--with-pcre=no - we do not want to use PCRE
condition: ||
"" (empty)	TRUE	TRUE	--with-pcre - we want to use PCRE
yes	TRUE	TRUE	--with-pcre=yes - we want to use PCRE
no	FALSE	FALSE	--with-pcre=no - we do not want to use PCRE

[BalkanMadman]

So, essentially, the only form of this --with-pcre option that should disable pcre is --with-pcre=no (--without-pcre desugars to --with-pcre=no), am I right? Any other forms of --with-pcre=blabla should be treated the same way as plain --with-pcre, correct? If so, I will change this PR accordingly.

[airween]

Hi @BalkanMadman,

first of all, many thanks for this PR. I think this is a really important patch, we must apply this.

Please take a look at my comment above - I don't expect you to review all conditions, but that would be a huge help for the project.

Also, could you take a look at the v2/master tree too, and make this modifications?