From 371ab7fa96f8e439f4008c973c37aa44ab6ed81e Mon Sep 17 00:00:00 2001
From: Robert Morris <rtm@csail.mit.edu>
Date: Thu, 1 Sep 2011 13:25:34 -0400
Subject: [PATCH] inaccessible page under the user stack page, to help exec
deal w/ too-large args
---
defs.h | 1 +
exec.c | 9 ++++++---
usertests.c | 2 +-
vm.c | 13 +++++++++++++
4 files changed, 21 insertions(+), 4 deletions(-)
diff --git a/defs.h b/defs.h
index 19d559b..4b3eec9 100644
--- a/defs.h
+++ b/defs.h
@@ -176,6 +176,7 @@ pde_t* copyuvm(pde_t*, uint);
void switchuvm(struct proc*);
void switchkvm(void);
int copyout(pde_t*, uint, void*, uint);
+void clear_pte_u(pde_t *pgdir, char *uva);
// number of elements in fixed-size array
#define NELEM(x) (sizeof(x)/sizeof((x)[0]))
diff --git a/exec.c b/exec.c
index dfff3e6..af9e006 100644
--- a/exec.c
+++ b/exec.c
@@ -49,13 +49,16 @@ exec(char *path, char **argv)
iunlockput(ip);
ip = 0;
- // Allocate a one-page stack at the next page boundary
+ // Allocate two pages at the next page boundary.
+ // Make the first inaccessible.
+ // Use the second as the user stack.
sz = PGROUNDUP(sz);
- if((sz = allocuvm(pgdir, sz, sz + PGSIZE)) == 0)
+ if((sz = allocuvm(pgdir, sz, sz + 2*PGSIZE)) == 0)
goto bad;
+ clear_pte_u(pgdir, (char*)(sz-2*PGSIZE));
+ sp = sz;
// Push argument strings, prepare rest of stack in ustack.
- sp = sz;
for(argc = 0; argv[argc]; argc++) {
if(argc >= MAXARG)
goto bad;
diff --git a/usertests.c b/usertests.c
index 62ce8d7..ed13d80 100644
--- a/usertests.c
+++ b/usertests.c
@@ -1525,7 +1525,7 @@ bigargtest(void)
for(i = 0; i < MAXARG-1; i++)
args[i] = "bigargs test: failed\n ";
args[MAXARG-1] = 0;
- printf(stdout, "bigarg test %d\n", (MAXARG-1)*strlen(args[0]));
+ printf(stdout, "bigarg test\n");
exec("echo", args);
printf(stdout, "bigarg test ok\n");
fd = open("bigarg-ok", O_CREATE);
diff --git a/vm.c b/vm.c
index 247fede..c717baf 100644
--- a/vm.c
+++ b/vm.c
@@ -363,3 +363,16 @@ copyout(pde_t *pgdir, uint va, void *p, uint len)
}
return 0;
}
+
+// Clear PTE_U on a page. Used to create an inaccessible
+// page beneath the user stack.
+void
+clear_pte_u(pde_t *pgdir, char *uva)
+{
+ pte_t *pte;
+
+ pte = walkpgdir(pgdir, uva, 0);
+ if(pte == 0)
+ panic("clear_pte_u");
+ *pte &= ~PTE_U;
+}
--
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