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SubjectRe: [PATCH 3/5] init: use do_mount() instead of ksys_mount()
On Thu, Dec 12, 2019 at 07:14:20PM +0100, Dominik Brodowski wrote:
> diff --git a/init/do_mounts.c b/init/do_mounts.c
> index 43f6d098c880..f55cbd9cb818 100644
> --- a/init/do_mounts.c
> +++ b/init/do_mounts.c
> @@ -387,12 +387,25 @@ static void __init get_fs_names(char *page)
> *s = '\0';
> }
>
> -static int __init do_mount_root(char *name, char *fs, int flags, void *data)
> +static int __init do_mount_root(const char *name, const char *fs,
> + const int flags, const void *data)
> {
> struct super_block *s;
> - int err = ksys_mount(name, "/root", fs, flags, data);
> - if (err)
> - return err;
> + char *data_page;
> + struct page *p;
> + int ret;
> +
> + /* do_mount() requires a full page as fifth argument */
> + p = alloc_page(GFP_KERNEL);
> + if (!p)
> + return -ENOMEM;
> +
> + data_page = page_address(p);
^^^^^^^^^^^^^^^^^^^^^^^^^^^^

That doesn't work in my guest as it gives a funny address:

[ 3.155314] mount_block_root: entry
[ 3.155868] mount_block_root: fs_name: [ext3]
[ 3.156512] do_mount_root: will copy data page: 0x00000000adf0ddb8

leading to the splat below.

Reverting the patch fixes the boot.

Thx.

[ 3.575074] BUG: kernel NULL pointer dereference, address: 0000000000000000
[ 3.576858] #PF: supervisor read access in kernel mode
[ 3.578274] #PF: error_code(0x0000) - not-present page
[ 3.579003] PGD 0 P4D 0
[ 3.579003] Oops: 0000 [#1] PREEMPT SMP
[ 3.579003] CPU: 8 PID: 1 Comm: swapper/0 Not tainted 5.5.0-rc1+ #17
[ 3.579003] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.11.1-1 04/01/2014
[ 3.579003] RIP: 0010:strncpy+0xf/0x30
[ 3.579003] Code: 0f b6 0c 16 88 0c 10 48 ff c2 84 c9 75 f2 f3 c3 66 66 2e 0f 1f 84 00 00 00 00 00 48 85 d2 48 89 f8 74 1b 4c 8d 04 17 48 89 fa <0f> b6 0e 80 f9 01 88 0a 48 83 de ff 48 ff c2 4c 39 c2 75 ec f3 c3
[ 3.579003] RSP: 0018:ffffc90000013eb8 EFLAGS: 00010206
[ 3.579003] RAX: ffff88807b780000 RBX: 0000000000008001 RCX: 0000000000000000
[ 3.579003] RDX: ffff88807b780000 RSI: 0000000000000000 RDI: ffff88807b780000
[ 3.579003] RBP: ffff88807b781000 R08: ffff88807b780fff R09: 00000000000770f4
[ 3.579003] R10: 0000000000000000 R11: 0000000000000000 R12: ffff88807b781000
[ 3.579003] R13: 0000000000000000 R14: 0000000000000000 R15: ffffea0001ede000
[ 3.579003] FS: 0000000000000000(0000) GS:ffff88807dc00000(0000) knlGS:0000000000000000
[ 3.579003] CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033
[ 3.579003] CR2: 0000000000000000 CR3: 0000000002009000 CR4: 00000000003406e0
[ 3.579003] Call Trace:
[ 3.579003] mount_block_root+0x14f/0x312
[ 3.579003] prepare_namespace+0x136/0x165
[ 3.579003] ? rest_init+0xb9/0xb9
[ 3.579003] kernel_init+0xa/0xf7
[ 3.579003] ret_from_fork+0x22/0x40
[ 3.579003] Modules linked in:
[ 3.579003] CR2: 0000000000000000
[ 3.579003] ---[ end trace 2884b7e501f1daa6 ]---
[ 3.579003] RIP: 0010:strncpy+0xf/0x30
[ 3.579003] Code: 0f b6 0c 16 88 0c 10 48 ff c2 84 c9 75 f2 f3 c3 66 66 2e 0f 1f 84 00 00 00 00 00 48 85 d2 48 89 f8 74 1b 4c 8d 04 17 48 89 fa <0f> b6 0e 80 f9 01 88 0a 48 83 de ff 48 ff c2 4c 39 c2 75 ec f3 c3
[ 3.579003] RSP: 0018:ffffc90000013eb8 EFLAGS: 00010206
[ 3.579003] RAX: ffff88807b780000 RBX: 0000000000008001 RCX: 0000000000000000
[ 3.579003] RDX: ffff88807b780000 RSI: 0000000000000000 RDI: ffff88807b780000
[ 3.579003] RBP: ffff88807b781000 R08: ffff88807b780fff R09: 00000000000770f4
[ 3.579003] R10: 0000000000000000 R11: 0000000000000000 R12: ffff88807b781000
[ 3.579003] R13: 0000000000000000 R14: 0000000000000000 R15: ffffea0001ede000
[ 3.579003] FS: 0000000000000000(0000) GS:ffff88807dc00000(0000) knlGS:0000000000000000
[ 3.579003] CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033
[ 3.579003] CR2: 0000000000000000 CR3: 0000000002009000 CR4: 00000000003406e0
[ 3.611795] Kernel panic - not syncing: Attempted to kill init! exitcode=0x00000009
[ 3.612923] Kernel Offset: disabled
[ 3.613505] ---[ end Kernel panic - not syncing: Attempted to kill init! exitcode=0x00000009 ]---

--
Regards/Gruss,
Boris.

https://people.kernel.org/tglx/notes-about-netiquette

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