fix importing weak symbols

README.md

@@ -36,6 +36,10 @@ ld -T ld/link.ld --oformat=binary -o output.elf nasm-output.o input.o...
   of glibc.
 * `NO_START_ARG`: *don't* pass the stack pointer to `_start` as the first arg.
   Will make it unable to read argc/argv/environ, but gives you 3 bytes.
+* `SKIP_ZERO_VALUE`: skip a `Sym` with a `st_value` field of `0`. If this isn't
+  enabled, weak symbols etc. might be imported instead of the real ones,
+  causing breakage. Many libraries don't have weak symbols at all, though.
+  Costs 4 (i386) or 5 (x86_64) bytes.
 
 ```
 usage: smol.py [-h] [-m TARGET] [-l LIB] [-L DIR] [--nasm NASM] [--cc CC]

rt/loader32.asm

@@ -133,6 +133,10 @@ _smol_start:
         .hasheq:
             mov eax, [edx + ST_VALUE_OFF]
             pop edx
+%ifdef SKIP_ZERO_VALUE
+             or eax, eax
+             jz short .next_link
+%endif
             mov esi, [edx + L_ADDR_OFF]
            ;cmp eax, esi
            ; jb short .hasheqnorel

rt/loader64.asm

@@ -127,6 +127,10 @@ _smol_start:
 
         .hasheq:
             mov rax, [rdx + ST_VALUE_OFF]
+%ifdef SKIP_ZERO_VALUE
+             or rax, rax
+             jz short .next_link
+%endif
             add rax, [r12 + L_ADDR_OFF]
           stosq
             cmp word [rdi], 0
@@ -215,6 +219,10 @@ repne scasd ; technically, scasq should be used, but meh. this is 1 byte smaller
                 ; ElfW(Addr) symoff(rax) = symtab[bucket].st_value
             lea rdx, [rcx + rcx * 2]
             mov rax, [rax + rdx * 8 + ST_VALUE_OFF]
+%ifdef SKIP_ZERO_VALUE
+             or rax, rax ; zero value => weak symbol or sth
+             jz short .next_link
+%endif
                 ; void* finaladdr(rax) = symoff + entry->l_addr
             add rax, [r12 + L_ADDR_OFF]