__init_libc: add fallbacks for __progname setup

It is possible for argv[0] to be a null pointer, but the __progname
variable is used to implement functions in src/legacy/err.c that do not
expect it to be null. It is also available to the user via the
program_invocation_name alias as a GNU extension, and the implementation
in Glibc initializes it to a pointer to empty string rather than NULL.

Since argv[0] is usually non-null and it's preferable to keep those
variables in BSS, implement the fallbacks in __init_libc, which also
allows to have an intermediate fallback to AT_EXECFN.

diff --git a/src/env/__libc_start_main.c b/src/env/__libc_start_main.c
index 5c79be2..18afdc1 100644 (file)
--- a/src/env/__libc_start_main.c
+++ b/src/env/__libc_start_main.c
@@ -30,10 +30,10 @@ void __init_libc(char **envp, char *pn)
        __sysinfo = aux[AT_SYSINFO];
        libc.page_size = aux[AT_PAGESZ];
 
-       if (pn) {
-               __progname = __progname_full = pn;
-               for (i=0; pn[i]; i++) if (pn[i]=='/') __progname = pn+i+1;
-       }
+       if (!pn) pn = (void*)aux[AT_EXECFN];
+       if (!pn) pn = "";
+       __progname = __progname_full = pn;
+       for (i=0; pn[i]; i++) if (pn[i]=='/') __progname = pn+i+1;
 
        __init_tls(aux);
        __init_ssp((void *)aux[AT_RANDOM]);