document and make explicit desired noinline property for __init_libc

on multiple occasions I've started to flatten/inline the code in
__init_libc, only to rediscover the reason it was not inlined: GCC
fails to deallocate its stack (and now, with the changes in commit
4390383b32250a941ec616e8bff6f568a801b1c0, fails to produce a tail call
to the stage 2 function; see PR #87639) before calling main if it was
inlined.

document this with a comment and use an explicit noinline attribute if
__GNUC__ is defined so that even with CFLAGS that heavily favor
inlining it won't get inlined.

diff --git a/src/env/__libc_start_main.c b/src/env/__libc_start_main.c
index ba4d213..2e5f9dc 100644 (file)
--- a/src/env/__libc_start_main.c
+++ b/src/env/__libc_start_main.c
@@ -17,6 +17,9 @@ weak_alias(dummy1, __init_ssp);
 
 #define AUX_CNT 38
 
+#ifdef __GNUC__
+__attribute__((__noinline__))
+#endif
 void __init_libc(char **envp, char *pn)
 {
        size_t i, *auxv, aux[AUX_CNT] = { 0 };
@@ -69,6 +72,9 @@ int __libc_start_main(int (*main)(int,char **,char **), int argc, char **argv)
 {
        char **envp = argv+argc+1;
 
+       /* External linkage, and explicit noinline attribute if available,
+        * are used to prevent the stack frame used during init from
+        * persisting for the entire process lifetime. */
        __init_libc(envp, argv[0]);
 
        /* Barrier against hoisting application code or anything using ssp