[src/trunk]: src/sys/kern execve_runproc: Unbreak __MACHINE_STACK_GROWS_UP ma...

[uebayasi <uebayasi%NetBSD.org@localhost>]

details:   https://anonhg.NetBSD.org/src/rev/9d47399094ac
branches:  trunk
changeset: 795415:9d47399094ac
user:      uebayasi <uebayasi%NetBSD.org@localhost>
date:      Sat Apr 12 05:25:23 2014 +0000

description:
execve_runproc: Unbreak __MACHINE_STACK_GROWS_UP machines.  Clarify the stack
address allocation code.  Summarize an awful big comment about the _rtld()
"gap".

(The log message in Rev. 1.384 was wrong; the new stack address is passed
not via the 3rd register argument, but via the SP.  The 3rd is for ps_strings.)

diffstat:

 sys/kern/kern_exec.c |  99 ++++++++++++++++++++++++++-------------------------
 1 files changed, 50 insertions(+), 49 deletions(-)

diffs (161 lines):

diff -r c4288d87e9c2 -r 9d47399094ac sys/kern/kern_exec.c
--- a/sys/kern/kern_exec.c      Fri Apr 11 18:02:33 2014 +0000
+++ b/sys/kern/kern_exec.c      Sat Apr 12 05:25:23 2014 +0000
@@ -1,4 +1,4 @@
-/*     $NetBSD: kern_exec.c,v 1.385 2014/04/11 18:02:33 uebayasi Exp $ */
+/*     $NetBSD: kern_exec.c,v 1.386 2014/04/12 05:25:23 uebayasi Exp $ */
 
 /*-
  * Copyright (c) 2008 The NetBSD Foundation, Inc.
@@ -59,7 +59,7 @@
  */
 
 #include <sys/cdefs.h>
-__KERNEL_RCSID(0, "$NetBSD: kern_exec.c,v 1.385 2014/04/11 18:02:33 uebayasi Exp $");
+__KERNEL_RCSID(0, "$NetBSD: kern_exec.c,v 1.386 2014/04/12 05:25:23 uebayasi Exp $");
 
 #include "opt_exec.h"
 #include "opt_execfmt.h"
@@ -243,6 +243,7 @@
        int                     ed_szsigcode;
        long                    ed_argc;
        long                    ed_envc;
+       vaddr_t                 ed_newstack;
 };
 
 /*
@@ -778,7 +779,7 @@
         * Calculate the new stack size.
         */
 
-       const size_t psstrauxlen =
+       const size_t nargenvptrs =
            data->ed_argc +             /* char *argv[] */
            1 +                         /* \0 */
            data->ed_envc +             /* char *env[] */
@@ -804,21 +805,31 @@
 #endif
 
 #ifdef __MACHINE_STACK_GROWS_UP
-/* See big comment lower down */
+/*
+ * copyargs() fills argc/argv/envp from the lower address even on
+ * __MACHINE_STACK_GROWS_UP machines.  Reserve a few words just below the SP
+ * so that _rtld() use it.
+ */
 #define        RTLD_GAP        32
 #else
 #define        RTLD_GAP        0
 #endif
 
-       const size_t stacklen =
+       const size_t argenvlen =
+           RTLD_GAP +                  /* reserved for _rtld() */
            sizeof(int) +               /* XXX argc in stack is long, not int */
-           (psstrauxlen * ptrsz) +     /* XXX auxinfo multiplied by ptr size? */
+           (nargenvptrs * ptrsz);      /* XXX auxinfo multiplied by ptr size? */
+
+       const size_t sigcode_psstr_sz =
+           data->ed_szsigcode +        /* sigcode */
+           data->ed_ps_strings_sz +    /* ps_strings */
+           STACK_PTHREADSPACE;         /* pthread space */
+
+       const size_t stacklen =
+           argenvlen +
            argenvstrlen +
            aslrgap +
-           RTLD_GAP +
-           data->ed_szsigcode +
-           data->ed_ps_strings_sz +
-           STACK_PTHREADSPACE;
+           sigcode_psstr_sz;
 
        /* make the stack "safely" aligned */
        const size_t aligned_stacklen = STACK_LEN_ALIGN(stacklen, STACK_ALIGNBYTES);
@@ -1117,39 +1128,36 @@
        data->ed_arginfo.ps_nenvstr = data->ed_envc;
 
     {
+       /*
+        * Allocate the stack address passed to the newly execve()'ed process.
+        *
+        * The new stack address will be set to the SP (stack pointer) register
+        * in setregs().
+        */
+
+       const size_t sigcode_psstr_sz =
+           data->ed_szsigcode +        /* sigcode */
+           data->ed_ps_strings_sz +    /* ps_strings */
+           STACK_PTHREADSPACE;         /* pthread space */
+
        char                    *stack;
 
-       stack = (char *)STACK_ALLOC(STACK_GROW(vm->vm_minsaddr,
-               STACK_PTHREADSPACE + data->ed_ps_strings_sz + data->ed_szsigcode),
-               epp->ep_ssize - (data->ed_ps_strings_sz + data->ed_szsigcode));
+       /* Top of the stack address space. */
+       stack = vm->vm_minsaddr;
+
+       /* Skip pthread space, ps_strings, and sigcode. */
+       stack = STACK_GROW(stack, sigcode_psstr_sz);
 
-#ifdef __MACHINE_STACK_GROWS_UP
-       /*
-        * The copyargs call always copies into lower addresses
-        * first, moving towards higher addresses, starting with
-        * the stack pointer that we give.  When the stack grows
-        * down, this puts argc/argv/envp very shallow on the
-        * stack, right at the first user stack pointer.
-        * When the stack grows up, the situation is reversed.
-        *
-        * Normally, this is no big deal.  But the ld_elf.so _rtld()
-        * function expects to be called with a single pointer to
-        * a region that has a few words it can stash values into,
-        * followed by argc/argv/envp.  When the stack grows down,
-        * it's easy to decrement the stack pointer a little bit to
-        * allocate the space for these few words and pass the new
-        * stack pointer to _rtld.  When the stack grows up, however,
-        * a few words before argc is part of the signal trampoline, XXX
-        * so we have a problem.
-        *
-        * Instead of changing how _rtld works, we take the easy way
-        * out and steal 32 bytes before we call copyargs.
-        * This extra space was allowed for when 'pack.ep_ssize' was calculated.
-        */
-       stack += RTLD_GAP;
-#endif /* __MACHINE_STACK_GROWS_UP */
-       
-       /* Now copy argc, args & environ to new stack */
+       /* Allocate the gap for _rtld() and arguments to be filled by copyargs(). */
+       stack = STACK_ALLOC(stack, epp->ep_ssize - sigcode_psstr_sz);
+
+       /* Skip a few words reserved for _rtld(). */
+       stack = STACK_GROW(stack, RTLD_GAP);
+
+       /* Remember the new stack address for setregs(). */
+       data->ed_newstack = (vaddr_t)stack;
+
+       /* Now copy argc, args & environ to the new stack. */
        error = (*epp->ep_esch->es_copyargs)(l, epp,
            &data->ed_arginfo, &stack, data->ed_argp);
 
@@ -1326,17 +1334,10 @@
 
        doexechooks(p);
 
-    {
-       char * const stack = (char *)STACK_GROW(
-           (void *)epp->ep_minsaddr, epp->ep_ssize);
-
        /* setup new registers and do misc. setup. */
-       (*epp->ep_esch->es_emul->e_setregs)(l, epp,
-            (vaddr_t)stack);
+       (*epp->ep_esch->es_emul->e_setregs)(l, epp, data->ed_newstack);
        if (epp->ep_esch->es_setregs)
-               (*epp->ep_esch->es_setregs)(l, epp,
-                   (vaddr_t)stack);
-    }
+               (*epp->ep_esch->es_setregs)(l, epp, data->ed_newstack);
 
        /* Provide a consistent LWP private setting */
        (void)lwp_setprivate(l, NULL);