obscure bug "extern void free (void *__ptr) __attribute__ ((__nothrow__ , __leaf__));"

[Mathias Steiger]

From: l0rd
To: bug-bash@gnu.org
Subject: obscure bug "extern void free (void *__ptr) __attribute__ 
((__nothrow__ , __leaf__));"

Configuration Information [Automatically generated, do not change]:
Machine: x86_64
OS: linux-gnu
Compiler: gcc
Compilation CFLAGS: -march=x86-64 -mtune=generic -O2 -pipe -fno-plt 
-DDEFAULT_PATH_VALUE='/usr/local/sbin:/usr/local/bin:/usr/bin' 
-DSTANDARD_UTILS_PATH='/usr/bin' -DSYS_BASHRC='/etc/bash.bashrc' 
-DSYS_BASH_LOGOUT='/etc/bash.bash_logout' -DNON_INTERACTIVE_LOGIN_SHELLS
uname output: Linux C1MPAQ 5.10.7-arch1-1 #1 SMP PREEMPT Wed, 13 Jan 
2021 12:02:01 +0000 x86_64 GNU/Linux
Machine Type: x86_64-pc-linux-gnu

Bash Version: 5.1 (Archlinux: core/bash 5.1.004-1)
Patch Level: 4
Release Status: release

Description:


An Autoconf configure script does fail, because some file it generated 
does unexpectedly contain output from some command it called in an 
if-statement that had output directed to >& /dev/null . Various 
alterations to the script do produce strange outcomes. Sometimes it 
avoids the bug, sometimes it will result in the line "extern void free 
(void *__ptr) __attribute__ ((__nothrow__ , __leaf__));" to be inserted 
into the file in addition to the behavior. This line is seemingly in 
some cases also be generated by the command itself that generated the 
misplaced output, which suggests that the bug is not in bash but some 
API bash is written in.


Repeat-By:


git clone https://github.com/LibreELEC/LibreELEC.tv
cd LibreELEC.tv
ARCH=aarch64 PROJECT=Amlogic DEVICE=AMLGX ./scripts/build linux
    -> the build fails after a minute in the Autoconfig step due to 
wrongful insertion of silenced command output into file config.status at 
line 533
In: build.LibreELEC-AMLGX.aarch64-9.80-devel/build/ccache-3.7.12/configure
Go to line 6532: if diff "$cache_file" confcache >/dev/null 2>&1; then 
:; else
Hint: $cache_file is always /dev/null , hence the if-statement will 
evaluate false

This diff command is the source of the insertion in 
build.LibreELEC-AMLGX.aarch64-9.80-devel/build/ccache-3.7.12/config.status :
        0a1,97:
        > # This file is a shell script that caches the results of 
configure
        > # tests run on this system so they can be shared between 
configure
        ...
Remove the line and the corresponding "fi" that closes the if-statement
    -> script inserts "extern void free ..." instead into 
./config.status at line 533
Replace line with: if diff "$cache_file" confcache >& /tmp/nothing; then 
:; else
    -> script inserts not only "extern void free ..." in config.status, 
but also the following into /tmp/nothing at the top in addition to the 
output:
        1c1,97
        < extern void free (void *__ptr) __attribute__ ((__nothrow__ , 
__leaf__));
        ---
        > # This file is a shell script that caches the results of 
configure
        ...
Replace line with if cat /tmp/nothing >& /dev/null; then :; else
    -> script inserts content of /tmp/nothing into ./config.status at 
line 533
Replace line with if false; then :; else # or other random commands
    -> script works


Content of /tmp/nothing (all characters are exact part of the file):
1c1,97
< extern void free (void *__ptr) __attribute__ ((__nothrow__ , __leaf__));
---
> # This file is a shell script that caches the results of configure
> # tests run on this system so they can be shared between configure
> # scripts and configure runs, see configure's option --config-cache.
> # It is not useful on other systems.  If it contains results you don't
> # want to keep, you may remove or edit it.
> #
> # config.status only pays attention to the cache file if you give it
> # the --recheck option to rerun configure.
> #
> # `ac_cv_env_foo' variables (set or unset) will be overridden when
> # loading this file, other *unset* `ac_cv_foo' will be assigned the
> # following values.
>
> ac_cv_build=${ac_cv_build=x86_64-pc-linux-gnu}
> ac_cv_c_bigendian=${ac_cv_c_bigendian=no}
> ac_cv_c_compiler_clang=${ac_cv_c_compiler_clang=no}
> ac_cv_c_compiler_gnu=${ac_cv_c_compiler_gnu=yes}
> ac_cv_c_extern_inline=${ac_cv_c_extern_inline=no}
> ac_cv_c_inline=${ac_cv_c_inline=inline}
> ac_cv_env_CC_set=set
> ac_cv_env_CC_value=/bin/gcc
> ac_cv_env_CFLAGS_set=set
> ac_cv_env_CFLAGS_value='-march=native -O2 -Wall -pipe 
-I/home/l0rd/LibreELEC.tv/build.LibreELEC-AMLGX.aarch64-9.80-devel/toolchain/include 
-Wno-format-security'
> ac_cv_env_CPPFLAGS_set=set
> ac_cv_env_CPPFLAGS_value=
> ac_cv_env_CPP_set=set
> ac_cv_env_CPP_value=cpp
> ac_cv_env_LDFLAGS_set=set
> 
ac_cv_env_LDFLAGS_value='-Wl,-rpath,/home/l0rd/LibreELEC.tv/build.LibreELEC-AMLGX.aarch64-9.80-devel/toolchain/lib 
-L/home/l0rd/LibreELEC.tv/build.LibreELEC-AMLGX.aarch64-9.80-devel/toolchain/lib'
> ac_cv_env_LIBS_set=
> ac_cv_env_LIBS_value=
> ac_cv_env_build_alias_set=set
> ac_cv_env_build_alias_value=x86_64-pc-linux-gnu
> ac_cv_env_host_alias_set=set
> ac_cv_env_host_alias_value=x86_64-pc-linux-gnu
> ac_cv_env_target_alias_set=
> ac_cv_env_target_alias_value=
> ac_cv_func_asprintf=${ac_cv_func_asprintf=yes}
> ac_cv_func_gethostname=${ac_cv_func_gethostname=yes}
> ac_cv_func_getopt_long=${ac_cv_func_getopt_long=yes}
> ac_cv_func_getpwuid=${ac_cv_func_getpwuid=yes}
> ac_cv_func_gettimeofday=${ac_cv_func_gettimeofday=yes}
> ac_cv_func_localtime_r=${ac_cv_func_localtime_r=yes}
> ac_cv_func_mkstemp=${ac_cv_func_mkstemp=yes}
> ac_cv_func_realpath=${ac_cv_func_realpath=yes}
> ac_cv_func_setenv=${ac_cv_func_setenv=yes}
> ac_cv_func_snprintf=${ac_cv_func_snprintf=yes}
> ac_cv_func_strndup=${ac_cv_func_strndup=yes}
> ac_cv_func_strtok_r=${ac_cv_func_strtok_r=yes}
> ac_cv_func_unsetenv=${ac_cv_func_unsetenv=yes}
> ac_cv_func_utimes=${ac_cv_func_utimes=yes}
> ac_cv_func_vasprintf=${ac_cv_func_vasprintf=yes}
> ac_cv_func_vsnprintf=${ac_cv_func_vsnprintf=yes}
> ac_cv_header_ctype_h=${ac_cv_header_ctype_h=yes}
> ac_cv_header_dirent_dirent_h=${ac_cv_header_dirent_dirent_h=yes}
> ac_cv_header_inttypes_h=${ac_cv_header_inttypes_h=yes}
> ac_cv_header_memory_h=${ac_cv_header_memory_h=yes}
> ac_cv_header_pwd_h=${ac_cv_header_pwd_h=yes}
> ac_cv_header_stdarg_h=${ac_cv_header_stdarg_h=yes}
> ac_cv_header_stdc=${ac_cv_header_stdc=yes}
> ac_cv_header_stdint_h=${ac_cv_header_stdint_h=yes}
> ac_cv_header_stdlib_h=${ac_cv_header_stdlib_h=yes}
> ac_cv_header_string_h=${ac_cv_header_string_h=yes}
> ac_cv_header_strings_h=${ac_cv_header_strings_h=yes}
> ac_cv_header_sys_mman_h=${ac_cv_header_sys_mman_h=yes}
> ac_cv_header_sys_stat_h=${ac_cv_header_sys_stat_h=yes}
> ac_cv_header_sys_time_h=${ac_cv_header_sys_time_h=yes}
> ac_cv_header_sys_types_h=${ac_cv_header_sys_types_h=yes}
> ac_cv_header_sys_wait_h=${ac_cv_header_sys_wait_h=yes}
> ac_cv_header_termios_h=${ac_cv_header_termios_h=yes}
> ac_cv_header_time=${ac_cv_header_time=yes}
> ac_cv_header_unistd_h=${ac_cv_header_unistd_h=yes}
> ac_cv_header_varargs_h=${ac_cv_header_varargs_h=no}
> ac_cv_host=${ac_cv_host=x86_64-pc-linux-gnu}
> ac_cv_objext=${ac_cv_objext=o}
> ac_cv_path_BASH=${ac_cv_path_BASH=/bin/sh}
> ac_cv_path_EGREP=${ac_cv_path_EGREP='/bin/grep -E'}
> ac_cv_path_GREP=${ac_cv_path_GREP=/bin/grep}
> ac_cv_path_install=${ac_cv_path_install='/bin/install -c'}
> ac_cv_prog_AR=${ac_cv_prog_AR=ar}
> ac_cv_prog_CC=${ac_cv_prog_CC=/bin/gcc}
> ac_cv_prog_CPP=${ac_cv_prog_CPP=cpp}
> ac_cv_prog_RANLIB=${ac_cv_prog_RANLIB=ranlib}
> ac_cv_prog_cc_c89=${ac_cv_prog_cc_c89=}
> ac_cv_prog_cc_c99=${ac_cv_prog_cc_c99=}
> ac_cv_prog_cc_g=${ac_cv_prog_cc_g=yes}
> ac_cv_search_cos=${ac_cv_search_cos=-lm}
> ac_cv_search_opendir=${ac_cv_search_opendir='none required'}
> ac_cv_type_long_long=${ac_cv_type_long_long=yes}
> 
ac_cv_type_unsigned_long_long_int=${ac_cv_type_unsigned_long_long_int=yes}
> ccache_cv_COMPAR_FN_T=${ccache_cv_COMPAR_FN_T=yes}
> hw_cv_func_asprintf=${hw_cv_func_asprintf=yes}
> hw_cv_func_snprintf=${hw_cv_func_snprintf=yes}
> hw_cv_func_snprintf_c99=${hw_cv_func_snprintf_c99=yes}
> hw_cv_func_vasprintf=${hw_cv_func_vasprintf=yes}
> hw_cv_func_vsnprintf=${hw_cv_func_vsnprintf=yes}
> hw_cv_func_vsnprintf_c99=${hw_cv_func_vsnprintf_c99=yes}