fs/select: avoid clang stack usage warning
diff mbox series

Message ID 20190307090146.1874906-1-arnd@arndb.de
State New
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Series
  • fs/select: avoid clang stack usage warning
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Commit Message

Arnd Bergmann March 7, 2019, 9:01 a.m. UTC
The select() implementation is carefully tuned to put a sensible amount
of data on the stack for holding a copy of the user space fd_set,
but not too large to risk overflowing the kernel stack.

When building a 32-bit kernel with clang, we need a little more space
than with gcc, which often triggers a warning:

fs/select.c:619:5: error: stack frame size of 1048 bytes in function 'core_sys_select' [-Werror,-Wframe-larger-than=]
int core_sys_select(int n, fd_set __user *inp, fd_set __user *outp,

I experimentally found that for 32-bit ARM, reducing the maximum
stack usage by 64 bytes keeps us reliably under the warning limit
again.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
---
 include/linux/poll.h | 4 ++++
 1 file changed, 4 insertions(+)

Comments

Andi Kleen March 7, 2019, 4:19 p.m. UTC | #1
On Thu, Mar 07, 2019 at 10:01:36AM +0100, Arnd Bergmann wrote:
> The select() implementation is carefully tuned to put a sensible amount
> of data on the stack for holding a copy of the user space fd_set,
> but not too large to risk overflowing the kernel stack.
> 
> When building a 32-bit kernel with clang, we need a little more space
> than with gcc, which often triggers a warning:
> 
> fs/select.c:619:5: error: stack frame size of 1048 bytes in function 'core_sys_select' [-Werror,-Wframe-larger-than=]
> int core_sys_select(int n, fd_set __user *inp, fd_set __user *outp,
> 
> I experimentally found that for 32-bit ARM, reducing the maximum
> stack usage by 64 bytes keeps us reliably under the warning limit
> again.

Could just use 768 bytes unconditionally. I doubt a few bytes more or less
will make too much difference.

Other than that

Reviewed-by: Andi Kleen <ak@linux.intel.com>

-Andi

Patch
diff mbox series

diff --git a/include/linux/poll.h b/include/linux/poll.h
index 7e0fdcf905d2..1cdc32b1f1b0 100644
--- a/include/linux/poll.h
+++ b/include/linux/poll.h
@@ -16,7 +16,11 @@ 
 extern struct ctl_table epoll_table[]; /* for sysctl */
 /* ~832 bytes of stack space used max in sys_select/sys_poll before allocating
    additional memory. */
+#ifdef __clang__
+#define MAX_STACK_ALLOC 768
+#else
 #define MAX_STACK_ALLOC 832
+#endif
 #define FRONTEND_STACK_ALLOC	256
 #define SELECT_STACK_ALLOC	FRONTEND_STACK_ALLOC
 #define POLL_STACK_ALLOC	FRONTEND_STACK_ALLOC