From 01711714d48e7c2383da109c6af92aebe552eae6 Mon Sep 17 00:00:00 2001 From: Ben Pfaff Date: Wed, 4 Jan 2006 22:45:01 +0000 Subject: [PATCH] Elaborate on how to get the user program's stack pointer. Suggested by Godmar Back. --- doc/vm.texi | 17 +++++++++++++++-- 1 file changed, 15 insertions(+), 2 deletions(-) diff --git a/doc/vm.texi b/doc/vm.texi index 982cdcb..28754c7 100644 --- a/doc/vm.texi +++ b/doc/vm.texi @@ -483,8 +483,7 @@ allocate additional pages as necessary. Allocate additional pages only if they ``appear'' to be stack accesses. Devise a heuristic that attempts to distinguish stack accesses from -other accesses. You can retrieve the user program's current stack -pointer from the @struct{intr_frame}'s @code{esp} member. +other accesses. User programs are buggy if they write to the stack below the stack pointer, because typical real OSes may interrupt a process at any time @@ -500,6 +499,20 @@ not be restartable in a straightforward fashion.) Similarly, the @code{PUSHA} instruction pushes 32 bytes at once, so it can fault 32 bytes below the stack pointer. +You will need to be able to obtain the current value of the user +program's stack pointer. Within a system call or a page fault generated +by a user program, you can retrieve it from @code{esp} member of the +@struct{intr_frame} passed to @func{syscall_handler} or +@func{page_fault}, respectively. If you verify user pointers before +accessing them (@pxref{Accessing User Memory}), these are the only cases +you need to handle. On the other hand, if you depend on page faults to +detect invalid memory access, you will need to handle another case, +where a page fault occurs in the kernel. Reading @code{esp} out of the +@struct{intr_frame} passed to @func{page_fault} in that case will obtain +the kernel stack pointer, not the user stack pointer. You will need to +arrange another way, e.g.@: by saving @code{esp} into @struct{thread} on +the initial transition from user to kernel mode. + You may impose some absolute limit on stack size, as do most OSes. Some OSes make the limit user-adjustable, e.g.@: with the @command{ulimit} command on many Unix systems. On many GNU/Linux systems, -- 2.30.2