Re: [PATCH] powerpc/64: Fix kernel stack 16-byte alignment

[Sandipan Das <sandipan@linux.ibm.com>]

On 15/11/18 8:04 AM, Nicholas Piggin wrote:
> Commit 4c2de74cc869 ("powerpc/64: Interrupts save PPR on stack rather
> than thread_struct") changed sizeof(struct pt_regs) % 16 from 0 to 8,
> which causes the interrupt frame allocation on kernel entry to put the
> kernel stack out of alignment.
> 
> Add a pad field to fix alignment, and add a BUILD_BUG_ON to catch this
> in future.
> 
> Fixes: 4c2de74cc869 ("powerpc/64: Interrupts save PPR on stack rather
> than thread_struct")
> Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
> [...]

Thanks for fixing this. Commit 4c2de74cc869 ("powerpc/64: Interrupts save
PPR on stack rather than thread_struct") was also leading to incorrect
kernel stack traces.

E.g.
If you are using `perf record -g` and expect to see a stack trace like this:

        c0000000002b19a0 bpf_check+0x1910 (/usr/lib/debug/lib/modules/4.18.17-200.fc28.ppc64le/vmlinux)
        c0000000002a5554 bpf_prog_load+0x684 (/usr/lib/debug/lib/modules/4.18.17-200.fc28.ppc64le/vmlinux)
        c0000000002a6938 sys_bpf+0xaf8 (/usr/lib/debug/lib/modules/4.18.17-200.fc28.ppc64le/vmlinux)
        c00000000000b9e4 system_call+0x5c (/usr/lib/debug/lib/modules/4.18.17-200.fc28.ppc64le/vmlinux)
            7fff815bca90 syscall+0x50 (/usr/lib64/libc-2.27.so)
            7fff7d96ee0c bpf_prog_load+0x16c (/usr/lib64/libbcc.so.0.7.0)
[...]

you would instead see something like this:

        c0000000002bdb88 bpf_check+0xb88 (/lib/modules/4.20.0-rc1+/build/vmlinux)
        c0000000002bdb60 bpf_check+0xb60 (/lib/modules/4.20.0-rc1+/build/vmlinux)
            3fff8f350a90 syscall+0x50 (/usr/lib64/libc-2.27.so)
            3fff8b788e0c bpf_prog_load+0x16c (/usr/lib64/libbcc.so.0.7.0)
[...]

--
With Regards,
Sandipan