[PATCH 1/1] parisc,metag: Do not hardcode maximum userspace stack size

[Helge Deller]

This patch affects only architectures where the stack grows upwards
(currently parisc and metag only). On those do not hardcode the maximum
initial stack size to 1GB for 32-bit processes, but make it configurable
via a config option.

The main problem with the hardcoded stack size is, that we have two
memory regions which grow upwards: stack and heap. To keep most of the
memory available for heap in a flexmap memoy layout, it makes no sense
to hard allocate up to 1GB of the memory for stack which can't be used
as heap then.

This patch makes the stack size configurable and uses 80MB as default
value which has been in use during the last few years on parisc and
which didn't showed any problems yet.

This also fixes a BUG on metag if the RLIMIT_STACK hard limit is
increased beyond a safe value by root. E.g. when starting a process
after running "ulimit -H -s unlimited" it will then attempt to use a
stack size of the maximum 1GB which is far too big for metag's limited
user virtual address space (stack_top is usually 0x3ffff000):
BUG: failure at fs/exec.c:589/shift_arg_pages()!

Signed-off-by: Helge Deller <deller@xxxxxx>
Signed-off-by: James Hogan <james.hogan@xxxxxxxxxx>
Cc: linux-parisc@xxxxxxxxxxxxxxx
Cc: linux-metag@xxxxxxxxxxxxxxx
Cc: John David Anglin <dave.anglin@xxxxxxxx>
Cc: stable@xxxxxxxxxxxxxxx # only needed for >= v3.9 (arch/metag)
---
v3 (James Hogan):
 - fix so that 64-bit parisc processes still use the 1GB limit.
   CONFIG_STACK_GROWSUP arches should provide a STACK_SIZE_MAX in their
   asm/processor.h, and for parisc it depends on USER_WIDE_MODE (whether
   the current process is 64-bit).
v2 (James Hogan):
 - updated description to mention BUG on metag.
 - added custom range limit for METAG.
 - moved Kconfig symbol to mm/Kconfig and reworded.
 - fixed "matag" typo.
---
 arch/metag/include/asm/processor.h  |  2 ++
 arch/parisc/include/asm/processor.h |  5 +++++
 arch/parisc/kernel/sys_parisc.c     |  6 +++---
 fs/exec.c                           |  6 +++---
 mm/Kconfig                          | 15 +++++++++++++++
 5 files changed, 28 insertions(+), 6 deletions(-)

diff --git a/arch/metag/include/asm/processor.h b/arch/metag/include/asm/processor.h
index f16477d1f571..a8a37477c66e 100644
--- a/arch/metag/include/asm/processor.h
+++ b/arch/metag/include/asm/processor.h
@@ -22,6 +22,8 @@
 /* Add an extra page of padding at the top of the stack for the guard page. */
 #define STACK_TOP	(TASK_SIZE - PAGE_SIZE)
 #define STACK_TOP_MAX	STACK_TOP
+/* Maximum virtual space for stack */
+#define STACK_SIZE_MAX	(CONFIG_MAX_STACK_SIZE_MB*1024*1024)

 /* This decides where the kernel will search for a free chunk of vm
  * space during mmap's.
diff --git a/arch/parisc/include/asm/processor.h b/arch/parisc/include/asm/processor.h
index 198a86feb574..d951c9681ab3 100644
--- a/arch/parisc/include/asm/processor.h
+++ b/arch/parisc/include/asm/processor.h
@@ -55,6 +55,11 @@
 #define STACK_TOP	TASK_SIZE
 #define STACK_TOP_MAX	DEFAULT_TASK_SIZE

+/* Allow bigger stacks for 64-bit processes */
+#define STACK_SIZE_MAX	(USER_WIDE_MODE					\
+			 ? (1 << 30)	/* 1 GB */			\
+			 : (CONFIG_MAX_STACK_SIZE_MB*1024*1024))
+
 #endif

 #ifndef __ASSEMBLY__
diff --git a/arch/parisc/kernel/sys_parisc.c b/arch/parisc/kernel/sys_parisc.c
index 31ffa9b55322..e1ffea2f9a0b 100644
--- a/arch/parisc/kernel/sys_parisc.c
+++ b/arch/parisc/kernel/sys_parisc.c
@@ -72,10 +72,10 @@ static unsigned long mmap_upper_limit(void)
 {
 	unsigned long stack_base;

-	/* Limit stack size to 1GB - see setup_arg_pages() in fs/exec.c */
+	/* Limit stack size - see setup_arg_pages() in fs/exec.c */
 	stack_base = rlimit_max(RLIMIT_STACK);
-	if (stack_base > (1 << 30))
-		stack_base = 1 << 30;
+	if (stack_base > STACK_SIZE_MAX)
+		stack_base = STACK_SIZE_MAX;

 	return PAGE_ALIGN(STACK_TOP - stack_base);
 }
diff --git a/fs/exec.c b/fs/exec.c
index 476f3ebf437e..238b7aa26f68 100644
--- a/fs/exec.c
+++ b/fs/exec.c
@@ -657,10 +657,10 @@ int setup_arg_pages(struct linux_binprm *bprm,
 	unsigned long rlim_stack;

 #ifdef CONFIG_STACK_GROWSUP
-	/* Limit stack size to 1GB */
+	/* Limit stack size */
 	stack_base = rlimit_max(RLIMIT_STACK);
-	if (stack_base > (1 << 30))
-		stack_base = 1 << 30;
+	if (stack_base > STACK_SIZE_MAX)
+		stack_base = STACK_SIZE_MAX;

 	/* Make sure we didn't let the argument array grow too large. */
 	if (vma->vm_end - vma->vm_start > stack_base)
diff --git a/mm/Kconfig b/mm/Kconfig
index ebe5880c29d6..1b5a95f0fa01 100644
--- a/mm/Kconfig
+++ b/mm/Kconfig
@@ -581,3 +581,18 @@ config PGTABLE_MAPPING

 config GENERIC_EARLY_IOREMAP
 	bool
+
+config MAX_STACK_SIZE_MB
+	int "Maximum user stack size for 32-bit processes (MB)"
+	default 80
+	range 8 256 if METAG
+	range 8 2048
+	depends on STACK_GROWSUP && (!64BIT || COMPAT)
+	help
+	  This is the maximum stack size in Megabytes in the VM layout of 32-bit
+	  user processes when the stack grows upwards (currently only on parisc
+	  and metag arch). The stack will be located at the highest memory
+	  address minus the given value, unless the RLIMIT_STACK hard limit is
+	  changed to a smaller value in which case that is used.
+
+	  A sane initial value is 80 MB.
-- 
1.9.3

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