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| H A D | process.c | diff d45c4b48dafb5820e5cc267ff9a6d7784d13a43c Thu Aug 24 08:42:10 CEST 2023 Michael Ellerman <mpe@ellerman.id.au> powerpc: Hide empty pt_regs at base of the stack
A thread started via eg. user_mode_thread() runs in the kernel to begin
with and then may later return to userspace. While it's running in the
kernel it has a pt_regs at the base of its kernel stack, but that
pt_regs is all zeroes.
If the thread oopses in that state, it leads to an ugly stack trace with
a big block of zero GPRs, as reported by Joel:
Kernel panic - not syncing: VFS: Unable to mount root fs on unknown-block(0,0)
CPU: 0 PID: 1 Comm: swapper/0 Not tainted 6.5.0-rc7-00004-gf7757129e3de-dirty #3
Hardware name: IBM PowerNV (emulated by qemu) POWER9 0x4e1200 opal:v7.0 PowerNV
Call Trace:
[c0000000036afb00] [c0000000010dd058] dump_stack_lvl+0x6c/0x9c (unreliable)
[c0000000036afb30] [c00000000013c524] panic+0x178/0x424
[c0000000036afbd0] [c000000002005100] mount_root_generic+0x250/0x324
[c0000000036afca0] [c0000000020057d0] prepare_namespace+0x2d4/0x344
[c0000000036afd20] [c0000000020049c0] kernel_init_freeable+0x358/0x3ac
[c0000000036afdf0] [c0000000000111b0] kernel_init+0x30/0x1a0
[c0000000036afe50] [c00000000000debc] ret_from_kernel_user_thread+0x14/0x1c
--- interrupt: 0 at 0x0
NIP: 0000000000000000 LR: 0000000000000000 CTR: 0000000000000000
REGS: c0000000036afe80 TRAP: 0000 Not tainted (6.5.0-rc7-00004-gf7757129e3de-dirty)
MSR: 0000000000000000 <> CR: 00000000 XER: 00000000
CFAR: 0000000000000000 IRQMASK: 0
GPR00: 0000000000000000 0000000000000000 0000000000000000 0000000000000000
GPR04: 0000000000000000 0000000000000000 0000000000000000 0000000000000000
GPR08: 0000000000000000 0000000000000000 0000000000000000 0000000000000000
GPR12: 0000000000000000 0000000000000000 0000000000000000 0000000000000000
GPR16: 0000000000000000 0000000000000000 0000000000000000 0000000000000000
GPR20: 0000000000000000 0000000000000000 0000000000000000 0000000000000000
GPR24: 0000000000000000 0000000000000000 0000000000000000 0000000000000000
GPR28: 0000000000000000 0000000000000000 0000000000000000 0000000000000000
NIP [0000000000000000] 0x0
LR [0000000000000000] 0x0
--- interrupt: 0
The all-zero pt_regs looks ugly and conveys no useful information, other
than its presence. So detect that case and just show the presence of the
frame by printing the interrupt marker, eg:
Kernel panic - not syncing: VFS: Unable to mount root fs on unknown-block(0,0)
CPU: 0 PID: 1 Comm: swapper/0 Not tainted 6.5.0-rc3-00126-g18e9506562a0-dirty #301
Hardware name: IBM pSeries (emulated by qemu) POWER9 (raw) 0x4e1202 0xf000005 of:SLOF,HEAD hv:linux,kvm pSeries
Call Trace:
[c000000003aabb00] [c000000001143db8] dump_stack_lvl+0x6c/0x9c (unreliable)
[c000000003aabb30] [c00000000014c624] panic+0x178/0x424
[c000000003aabbd0] [c0000000020050fc] mount_root_generic+0x250/0x324
[c000000003aabca0] [c0000000020057cc] prepare_namespace+0x2d4/0x344
[c000000003aabd20] [c0000000020049bc] kernel_init_freeable+0x358/0x3ac
[c000000003aabdf0] [c0000000000111b0] kernel_init+0x30/0x1a0
[c000000003aabe50] [c00000000000debc] ret_from_kernel_user_thread+0x14/0x1c
--- interrupt: 0 at 0x0
To avoid ever suppressing a valid pt_regs make sure the pt_regs has a
zero MSR and TRAP value, and is located at the very base of the stack.
Fixes: 6895dfc04741 ("powerpc: copy_thread fill in interrupt frame marker and back chain")
Reported-by: Joel Stanley <joel@jms.id.au>
Reported-by: Nicholas Piggin <npiggin@gmail.com>
Reviewed-by: Joel Stanley <joel@jms.id.au>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://msgid.link/20230824064210.907266-1-mpe@ellerman.id.au
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