LLVM cannot find clang binary - clang

Question

I have just built and installed LLVM Clang 3.5.0 with compiler-rt. clang binary seems to work, but cannot build a simple test program:
$ cat hello.c
#include <stdio.h>
int main(int argc, char **argv) {
printf("Hello World\n");
return 0;
}
Building it borks with error: unable to execute command: Executable "" doesn't exist!
$ clang hello.c -o hello
error: unable to execute command: Executable "" doesn't exist!
Executable "" ? Interesting...
Debugging it further reveals that clang tries to call itself to build .o object file and then ld to link it, but does not know where itself exists apparently.
$ clang -### hello.c -o hello
clang version 3.5.0 (tags/RELEASE_350/final)
Target: x86_64-alpine-linux-musl
Thread model: posix
"" "-cc1" "-triple" "x86_64-alpine-linux-musl" "-emit-obj" "-mrelax-all" "-disable-free" "-main-file-name" "hello.c" "-mrelocation-model" "static" "-mdisable-fp-elim" "-fmath-errno" "-masm-verbose" "-mconstructor-aliases" "-munwind-tables" "-fuse-init-array" "-target-cpu" "x86-64" "-target-linker-version" "2.24" "-dwarf-column-info" "-resource-dir" "../lib/clang/3.5.0" "-internal-isystem" "/usr/local/include" "-internal-isystem" "../lib/clang/3.5.0/include" "-internal-externc-isystem" "/include" "-internal-externc-isystem" "/usr/include" "-fdebug-compilation-dir" "/" "-ferror-limit" "19" "-fmessage-length" "158" "-mstackrealign" "-fobjc-runtime=gcc" "-fdiagnostics-show-option" "-o" "/tmp/hello-37746e.o" "-x" "c" "hello.c"
"/usr/bin/ld" "-z" "relro" "--eh-frame-hdr" "-m" "elf_x86_64" "-dynamic-linker" "/lib/ld-musl-x86_64.so.1" "-o" "hello" "/usr/bin/../lib/gcc/x86_64-alpine-linux-musl/4.8.3/../../../crt1.o" "/usr/bin/../lib/gcc/x86_64-alpine-linux-musl/4.8.3/../../../crti.o" "/usr/bin/../lib/gcc/x86_64-alpine-linux-musl/4.8.3/crtbegin.o" "-L/usr/bin/../lib/gcc/x86_64-alpine-linux-musl/4.8.3" "-L/usr/bin/../lib/gcc/x86_64-alpine-linux-musl/4.8.3/../../../../x86_64-alpine-linux-musl/lib" "-L/usr/bin/../lib/gcc/x86_64-alpine-linux-musl/4.8.3/../../.." "-L/../lib" "-L/lib" "-L/usr/lib" "/tmp/hello-37746e.o" "-lgcc" "--as-needed" "-lgcc_s" "--no-as-needed" "-lc" "-lgcc" "--as-needed" "-lgcc_s" "--no-as-needed" "/usr/bin/../lib/gcc/x86_64-alpine-linux-musl/4.8.3/crtend.o" "/usr/bin/../lib/gcc/x86_64-alpine-linux-musl/4.8.3/../../../crtn.o"
When I run the first line putting /usr/bin/clang as the first item, it builds just fine:
$ /usr/bin/clang "-cc1" "-triple" "x86_64-alpine-linux-musl" "-emit-obj" "-mrelax-all" "-disable-free" "-main-file-name" "hello.c" "-mrelocation-model" "static" "-mdisable-fp-elim" "-fmath-errno" "-masm-verbose" "-mconstructor-aliases" "-munwind-tables" "-target-cpu" "x86-64" "-target-linker-version" "2.24" "-dwarf-column-info" "-resource-dir" "../lib/clang/3.5.0" "-internal-isystem" "/usr/local/include" "-internal-isystem" "../lib/clang/3.5.0/include" "-internal-externc-isystem" "/include" "-internal-externc-isystem" "/usr/include" "-fdebug-compilation-dir" "/" "-ferror-limit" "19" "-fmessage-length" "158" "-mstackrealign" "-fobjc-runtime=gcc" "-fdiagnostics-show-option" "-o" "/tmp/hello-4f64bb.o" "-x" "c" "hello.c"
$
And following /usr/bin/ld is able to link it just fine resulting in:
$ ./hello
Hello World
Anny suggestions what did I screw during configure/build?

Answer 1

clang source analysis shows that 'clang' program on Linux uses /proc/self/exe to find out real path of its binary. I am running in chroot without /proc mounted, thus it failed.
mount -t proc proc /proc
solves the issue