Re: [Libguestfs] [libnbd PATCH v4 3/3] lib/utils: add unit test for async-signal-safe assert()

On 3/15/23 18:25, Eric Blake wrote: *shudder* Another hideous piece of complexity that's orthogonal to what one wants to do :) So, I investigated a bit. If I understand correctly, your point is that we could get a *false negative* here, because AC_CHECK_FUNCS might not find prctl() due to the autoconf-generated test program not #including <sys/prctl.h>. Assuming I got that right, I have two comments on it: (1) A false negative in this case would not be a huge problem; we'd miss out on prctl(), i.e. the test program would remain dumpable on Linux. The test would still function as needed, just litter the user's machine with a coredump during "make check". Not ideal, but also not tragic. (2) I believe I disagree with the idea that AC_CHECK_FUNCS might not find an otherwise existent prctl() *due to* AC_CHECK_FUNCS not generating "#include <sys/prctl.h>" into the test program. On my RHEL-9.1 install (using autoconf-2.69-38.el9.noarch), I've checked the generated "configure" script. First, we have 18509 for ac_func in \ 18510 posix_fadvise \ 18511 posix_memalign \ 18512 prctl \ 18513 strerrordesc_np \ 18514 valloc 18515 do : 18516 as_ac_var=`$as_echo "ac_cv_func_$ac_func" | $as_tr_sh` 18517 ac_fn_c_check_func "$LINENO" "$ac_func" "$as_ac_var" 18518 if eval test \"x\$"$as_ac_var"\" = x"yes"; then : 18519 cat >>confdefs.h <<_ACEOF 18520 #define `$as_echo "HAVE_$ac_func" | $as_tr_cpp` 1 18521 _ACEOF 18522 18523 fi 18524 done I.e., we call "ac_fn_c_check_func" with "prctl" passed as second argument. Then, that function is defined as follows: 2001 # ac_fn_c_check_func LINENO FUNC VAR 2002 # ---------------------------------- 2003 # Tests whether FUNC exists, setting the cache variable VAR accordingly 2004 ac_fn_c_check_func () 2005 { 2006 as_lineno=${as_lineno-"$1"} as_lineno_stack=as_lineno_stack=$as_lineno_stack 2007 { $as_echo "$as_me:${as_lineno-$LINENO}: checking for $2" >&5 2008 $as_echo_n "checking for $2... " >&6; } 2009 if eval \${$3+:} false; then : 2010 $as_echo_n "(cached) " >&6 2011 else 2012 cat confdefs.h - <<_ACEOF >conftest.$ac_ext 2013 /* end confdefs.h. */ 2014 /* Define $2 to an innocuous variant, in case <limits.h> declares $2. 2015 For example, HP-UX 11i <limits.h> declares gettimeofday. */ 2016 #define $2 innocuous_$2 2017 2018 /* System header to define __stub macros and hopefully few prototypes, 2019 which can conflict with char $2 (); below. 2020 Prefer <limits.h> to <assert.h> if __STDC__ is defined, since 2021 <limits.h> exists even on freestanding compilers. */ 2022 2023 #ifdef __STDC__ 2024 # include <limits.h> 2025 #else 2026 # include <assert.h> 2027 #endif 2028 2029 #undef $2 2030 2031 /* Override any GCC internal prototype to avoid an error. 2032 Use char because int might match the return type of a GCC 2033 builtin and then its argument prototype would still apply. */ 2034 #ifdef __cplusplus 2035 extern "C" 2036 #endif 2037 char $2 (); 2038 /* The GNU C library defines this for functions which it implements 2039 to always fail with ENOSYS. Some functions are actually named 2040 something starting with __ and the normal name is an alias. */ 2041 #if defined __stub_$2 || defined __stub___$2 2042 choke me 2043 #endif 2044 2045 int 2046 main () 2047 { 2048 return $2 (); 2049 ; 2050 return 0; 2051 } 2052 _ACEOF 2053 if ac_fn_c_try_link "$LINENO"; then : 2054 eval "$3=yes" 2055 else 2056 eval "$3=no" 2057 fi 2058 rm -f core conftest.err conftest.$ac_objext \ 2059 conftest$ac_exeext conftest.$ac_ext 2060 fi 2061 eval ac_res=\$$3 2062 { $as_echo "$as_me:${as_lineno-$LINENO}: result: $ac_res" >&5 2063 $as_echo "$ac_res" >&6; } 2064 eval $as_lineno_stack; ${as_lineno_stack:+:} unset as_lineno 2065 2066 } # ac_fn_c_check_func As far as I can tell, the test program provides its own declaration for prctl() on line 2037, so it does not depend on any system header providing a real prototype. ... I figured autoconf should have a "header check" too, and indeed it does: AC_CHECK_HEADER, AC_CHECK_HEADERS, at <https://www.gnu.org/software/autoconf/manual/autoconf-2.69/html_node/Gene...;. We do use AC_CHECK_HEADERS in libnbd. The question is now whether checking for <sys/prctl.h> with AC_CHECK_HEADERS is "more robust" than checking for prctl() with AC_CHECK_FUNCS(). I'd say: AC_CHECK_FUNCS() is a tiny bit stronger (we actually want to be able to call the particular prctl() function), but they should be mostly *interchangeable*. I'm saying that because prctl() is "Linux-specific" (per prctl(2) manual), so: (a) <sys/prctl.h> existing (per AC_CHECK_HEADERS), but not exposing the real -- and linkable -- prctl() prototype, (b) a call to prctl() being linkable (via the bogus declaration used by AC_CHECK_HEADERS), but <sys/prctl.h> not exposing a proper prctl() prototype, are *equally* Linux userspace bugs. So indeed I'll stick with the AC_CHECK_FUNCS approach. Thank you! Laszlo