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Date:   Thu, 21 Apr 2022 22:47:01 +0200
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From:   =?UTF-8?Q?Mateusz_Jo=c5=84czyk?= <mat.jonczyk@o2.pl>
Subject: "mm: uninline copy_overflow()" breaks i386 build in Mellanox MLX4
Cc:     David Laight <David.Laight@ACULAB.COM>,
        Andrew Morton <akpm@linux-foundation.org>,
        Christophe Leroy <christophe.leroy@csgroup.eu>,
        Anshuman Khandual <anshuman.khandual@arm.com>,
        linux-rdma@vger.kernel.org
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Hello,

commit ad7489d5262d ("mm: uninline copy_overflow()")

breaks for me a build for i386 in the Mellanox MLX4 driver:

        In file included from ./arch/x86/include/asm/preempt.h:7,
                         from ./include/linux/preempt.h:78,
                         from ./include/linux/percpu.h:6,
                         from ./include/linux/context_tracking_state.h:5,
                         from ./include/linux/hardirq.h:5,
                         from drivers/net/ethernet/mellanox/mlx4/cq.c:37:
        In function ‘check_copy_size’,
            inlined from ‘copy_to_user’ at ./include/linux/uaccess.h:159:6,
            inlined from ‘mlx4_init_user_cqes’ at drivers/net/ethernet/mellanox/mlx4/cq.c:317:9,
            inlined from ‘mlx4_cq_alloc’ at drivers/net/ethernet/mellanox/mlx4/cq.c:394:10:
        ./include/linux/thread_info.h:228:4: error: call to ‘__bad_copy_from’ declared with attribute error: copy source size is too small
          228 |    __bad_copy_from();
              |    ^~~~~~~~~~~~~~~~~
        make[5]: *** [scripts/Makefile.build:288: drivers/net/ethernet/mellanox/mlx4/cq.o] Błąd 1
        make[4]: *** [scripts/Makefile.build:550: drivers/net/ethernet/mellanox/mlx4] Błąd 2
        make[3]: *** [scripts/Makefile.build:550: drivers/net/ethernet/mellanox] Błąd 2
        make[2]: *** [scripts/Makefile.build:550: drivers/net/ethernet] Błąd 2
        make[1]: *** [scripts/Makefile.build:550: drivers/net] Błąd 2

Reverting this commit fixes the build. Disabling Mellanox Ethernet drivers
in Kconfig (tested only with also disabling of all Infiniband support) also fixes the build.

It appears that uninlining of copy_overflow() causes GCC to analyze the code deeper.

The code in mlx4_init_user_cqes, for reference:

        static int mlx4_init_user_cqes(void *buf, int entries, int cqe_size)
        {
                int entries_per_copy = PAGE_SIZE / cqe_size;
                void *init_ents;
                int err = 0;
                int i;

                init_ents = kmalloc(PAGE_SIZE, GFP_KERNEL);
                // ...
                if (entries_per_copy < entries) {
                        // ...
                } else {
                        // BUG here
                        err = copy_to_user((void __user *)buf, init_ents,
                                        array_size(entries, cqe_size)) ?
                                -EFAULT : 0;
                }

                // ...
        }

My setup: Ubuntu 20.04, gcc version 9.4.0 (Ubuntu 9.4.0-1ubuntu1~20.04.1)

I was using lightly modified Kconfig from Debian i386 Linux packages.

Greetings,

Mateusz Jończyk