error: performing pointer subtraction with a null pointer has undefined behavior [-Werror,-Wnull-pointer-subtraction]

[jaygooby]

• How to trigger this bug?

Building nginx on MacOS 12.3.1 with Xcode gcc Apple clang version 13.1.6 (clang-1316.0.21.2.5), nginx 1.20.1 and ngx_waf current branch.

I downloaded the nginx and ngx_waf sources. Using the current branch of ngx_waf I added cjson and uthash:

cd ngx_waf_current
git clone -b v2.3.0 https://github.com/troydhanson/uthash.git lib/uthash
git clone -b v1.7.15 https://github.com/DaveGamble/cJSON.git lib/cjson 

nginx configures OK, but make generates these errors (and many more that I haven't pasted here for sake of clarity):

-o objs/addon/src/ngx_http_waf_module_config.o \
/tmp/nw/ngx_waf-current/src/ngx_http_waf_module_config.c
/tmp/nw/ngx_waf-current/src/ngx_http_waf_module_config.c:92:25: error: performing pointer subtraction with a null pointer has undefined behavior [-Werror,-Wnull-pointer-subtraction]
p = (ngx_str_t*)utarray_next(array, NULL);
        ^~~~~~~~~~~~~~~~~~~~~~~~~
/tmp/nw/ngx_waf-current/lib/uthash/include/utarray.h:228:73: note: expanded from macro 'utarray_next'
#define utarray_next(a,e) (((e)==NULL) ? utarray_front(a) : (((a)->i != utarray_eltidx(a,e)+1) ? _utarray_eltptr(a,utarray_eltidx(a,e)+1) : NULL))
                                                        ^~~~~~~~~~~~~~~~~~~
/tmp/nw/ngx_waf-current/lib/uthash/include/utarray.h:231:42: note: expanded from macro 'utarray_eltidx'
#define utarray_eltidx(a,e) (((char*)(e) - (a)->d) / (a)->icd.sz)
              ~~~~~~~~~~ ^
/tmp/nw/ngx_waf-current/src/ngx_http_waf_module_config.c:92:25: error: performing pointer subtraction with a null pointer has undefined behavior [-Werror,-Wnull-pointer-subtraction]
p = (ngx_str_t*)utarray_next(array, NULL);
        ^~~~~~~~~~~~~~~~~~~~~~~~~

Which I guess are coming from the uthash library?

• The version or branch of ngx_waf. current

• Output of nginx -V.

  It doesn't compile, but this is how it was configured:

  LIB_MODSECURITY=/opt/modsecurity ./configure --add-module=/tmp/nw/ngx_waf-current

Optional Info

MacOS 12.3.1

• Whether nginx is running in a virtual environment such as Docker. If it is running in Docker, please provide the image name.

Not running under Docker

[jaygooby]

If I remove -Werror from the nginx objs/Makefile then it will compile

[ADD-SP]

It looks like the compiler's dataflow analysis is behaving differently, not a bug.

[stale[bot]]

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