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inaccessible page under the user stack page, to help exec deal w/ too-large args

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Robert Morris 2011-09-01 13:25:34 -04:00
parent 62e3b8a92c
commit 371ab7fa96
4 changed files with 21 additions and 4 deletions
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1
defs.h
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@ -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]))

9
exec.c
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@ -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;

2
usertests.c
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@ -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);

13
vm.c
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@ -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;
}