[patch] UML kernels on {i386,x86_64} produce bad coredumps

From: Paul Pluzhnikov
Date: Tue Jan 25 2011 - 18:15:06 EST


One of our users reported that when a user-level program SIGSEGVs under
UML kernel, the resulting core dump is not very usable.

I have reproduced that with the latest kernel:

make ARCH=um defconfig; make ARCH=um

Run the resulting kernel, then "inside" run this program:

#include <pthread.h>

void *fn(void *p)

int main()
pthread_t tid;
pthread_create(&tid, 0, fn, 0);
pthread_join(tid, 0);
return 0;

Analyze the coredump with GDB. Here is what you'll see:

sudo gdb -q -ex 'set solib-absolute-prefix ../root_fs' -ex 'file ../root_fs/var/tmp/mt-abort' -ex 'core ../root_fs/var/tmp/core.762'
Reading symbols from /usr/local/google/root_fs/var/tmp/mt-abort...done.
[New Thread 763]
[New Thread 762]
Core was generated by `./mt-abort'.
Program terminated with signal 6, Aborted.
#0 0x0000000040255250 in raise () from ../root_fs/lib64/libc.so.6
(gdb) info thread
2 Thread 762 0x0000000000000000 in ?? ()
* 1 Thread 763 0x0000000040255250 in raise () from ../root_fs/lib64/libc.so.6

Note that thread#2 looks funny.

(gdb) thread 2
[Switching to thread 2 (Thread 762)]#0 0x0000000000000000 in ?? ()
(gdb) info reg
rax 0x0 0
rbx 0x0 0
rcx 0x0 0
rdx 0x0 0
rsi 0x0 0
rdi 0x0 0
rbp 0x0 0x0
rsp 0x0 0x0
r8 0x0 0
r9 0x0 0
r10 0x0 0
r11 0x0 0
r12 0x0 0
r13 0x0 0
r14 0x0 0
r15 0x0 0
rip 0x0 0
eflags 0x0 [ ]
cs 0x0 0
ss 0x0 0
ds 0x0 0
es 0x0 0
fs 0x0 0
gs 0x0 0

Examining the core shows that NT_PRSTATUS notes for all threads other than
the one that crashed are zeroed out.

I believe this is happening because neither ELF_CORE_COPY_TASK_REGS nor
task_pt_regs are defined under ARCH=um, and so elf_core_copy_task_regs()
becomes a no-op.

Attached patch fixes this for SUBARCH={x86_64,i386}.

Paul Pluzhnikov

P.S. Google has blanket FSF copyright assignment.

Signed-off-by: Paul Pluzhnikov <ppluzhnikov@xxxxxxxxxx>

diff --git a/arch/um/sys-i386/asm/elf.h b/arch/um/sys-i386/asm/elf.h
index a979a22..d964a41 100644
--- a/arch/um/sys-i386/asm/elf.h
+++ b/arch/um/sys-i386/asm/elf.h
@@ -75,6 +75,8 @@ typedef struct user_i387_struct elf_fpregset_t;
pr_reg[16] = PT_REGS_SS(regs); \
} while (0);

+#define task_pt_regs(t) (&(t)->thread.regs)
struct task_struct;

extern int elf_core_copy_fpregs(struct task_struct *t, elf_fpregset_t *fpu);
diff --git a/arch/um/sys-x86_64/asm/elf.h b/arch/um/sys-x86_64/asm/elf.h
index d760967f..d6d5af3 100644
--- a/arch/um/sys-x86_64/asm/elf.h
+++ b/arch/um/sys-x86_64/asm/elf.h
@@ -95,6 +95,8 @@ typedef struct user_i387_struct elf_fpregset_t;
(pr_reg)[25] = 0; \
(pr_reg)[26] = 0;

+#define task_pt_regs(t) (&(t)->thread.regs)
struct task_struct;

extern int elf_core_copy_fpregs(struct task_struct *t, elf_fpregset_t *fpu);
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