[PATCH 4.14 05/71] fs/select: avoid clang stack usage warning

[Greg Kroah-Hartman]

From: Arnd Bergmann <arnd@xxxxxxxx>

commit ad312f95d41c9de19313c51e388c4984451c010f upstream.

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.

Link: http://lkml.kernel.org/r/20190307090146.1874906-1-arnd@xxxxxxxx
Signed-off-by: Arnd Bergmann <arnd@xxxxxxxx>
Reviewed-by: Andi Kleen <ak@xxxxxxxxxxxxxxx>
Cc: Nick Desaulniers <ndesaulniers@xxxxxxxxxx>
Cc: Alexander Viro <viro@xxxxxxxxxxxxxxxxxx>
Cc: Christoph Hellwig <hch@xxxxxx>
Cc: Eric Dumazet <edumazet@xxxxxxxxxx>
Cc: "Darrick J. Wong" <darrick.wong@xxxxxxxxxx>
Cc: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
Signed-off-by: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
Signed-off-by: Miles Chen <miles.chen@xxxxxxxxxxxx>
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>

---
 include/linux/poll.h |    4 ++++
 1 file changed, 4 insertions(+)

--- a/include/linux/poll.h
+++ b/include/linux/poll.h
@@ -15,7 +15,11 @@
 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