linux-kernel - [PATCH] fs/select: avoid clang stack usage warning

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Date:   Thu,  7 Mar 2019 10:01:36 +0100
From:   Arnd Bergmann <arnd@...db.de>
To:     linux-fsdevel@...r.kernel.org
Cc:     Nick Desaulniers <ndesaulniers@...gle.com>,
        Alexander Viro <viro@...iv.linux.org.uk>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Andi Kleen <ak@...ux.intel.com>,
        Christoph Hellwig <hch@....de>,
        Eric Dumazet <edumazet@...gle.com>,
        Arnd Bergmann <arnd@...db.de>,
        "Darrick J. Wong" <darrick.wong@...cle.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        linux-kernel@...r.kernel.org
Subject: [PATCH] fs/select: avoid clang stack usage warning

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

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
-- 
2.20.0