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From:   Greg Kroah-Hartman <gregkh@linuxfoundation.org>
To:     linux-kernel@vger.kernel.org
Cc:     Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        stable@vger.kernel.org, Arnd Bergmann <arnd@arndb.de>,
        Andi Kleen <ak@linux.intel.com>,
        Nick Desaulniers <ndesaulniers@google.com>,
        Alexander Viro <viro@zeniv.linux.org.uk>,
        Christoph Hellwig <hch@lst.de>,
        Eric Dumazet <edumazet@google.com>,
        "Darrick J. Wong" <darrick.wong@oracle.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Miles Chen <miles.chen@mediatek.com>
Subject: [PATCH 4.9 05/97] fs/select: avoid clang stack usage warning
Date:   Wed, 22 Jan 2020 10:28:09 +0100
Message-Id: <20200122092756.601582145@linuxfoundation.org>
In-Reply-To: <20200122092755.678349497@linuxfoundation.org>
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From: Arnd Bergmann <arnd@arndb.de>

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@arndb.de
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Reviewed-by: Andi Kleen <ak@linux.intel.com>
Cc: Nick Desaulniers <ndesaulniers@google.com>
Cc: Alexander Viro <viro@zeniv.linux.org.uk>
Cc: Christoph Hellwig <hch@lst.de>
Cc: Eric Dumazet <edumazet@google.com>
Cc: "Darrick J. Wong" <darrick.wong@oracle.com>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Miles Chen <miles.chen@mediatek.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

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

--- a/include/linux/poll.h
+++ b/include/linux/poll.h
@@ -14,7 +14,11 @@
 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