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Subject[PATCH 1/5] uml: fix random segfaults at bootup
Date

Don't use printk() where "current_thread_info()" is crap.

Until when we switch to running on init_stack, current_thread_info() evaluates
to crap. Printk uses "current" at times (in detail, &current is evaluated with
CONFIG_DEBUG_SPINLOCK to check the spinlock owner task).

And this leads to random segmentation faults.

Exactly, what happens is that &current = *(current_thread_info()), i.e. round
down $esp and dereference the value. I.e. access the stack below $esp, which
causes SIGSEGV on a VM_GROWSDOWN vma (see arch/i386/mm/fault.c).

Signed-off-by: Paolo 'Blaisorblade' Giarrusso <blaisorblade@yahoo.it>
---
arch/um/os-Linux/start_up.c | 22 ++++++++++++----------
1 files changed, 12 insertions(+), 10 deletions(-)
diff --git a/arch/um/os-Linux/start_up.c b/arch/um/os-Linux/start_up.c
index 37517d4..29a9e3f 100644
--- a/arch/um/os-Linux/start_up.c
+++ b/arch/um/os-Linux/start_up.c
@@ -116,16 +116,16 @@ static int stop_ptraced_child(int pid, v
if(!WIFEXITED(status) || (WEXITSTATUS(status) != exitcode)) {
int exit_with = WEXITSTATUS(status);
if (exit_with == 2)
- printk("check_ptrace : child exited with status 2. "
+ printf("check_ptrace : child exited with status 2. "
"Serious trouble happening! Try updating your "
"host skas patch!\nDisabling SYSEMU support.");
- printk("check_ptrace : child exited with exitcode %d, while "
+ printf("check_ptrace : child exited with exitcode %d, while "
"expecting %d; status 0x%x", exit_with,
exitcode, status);
if (mustpanic)
panic("\n");
else
- printk("\n");
+ printf("\n");
ret = -1;
}

@@ -183,7 +183,7 @@ static void __init check_sysemu(void)
void *stack;
int pid, n, status, count=0;

- printk("Checking syscall emulation patch for ptrace...");
+ printf("Checking syscall emulation patch for ptrace...");
sysemu_supported = 0;
pid = start_ptraced_child(&stack);

@@ -207,10 +207,10 @@ static void __init check_sysemu(void)
goto fail_stopped;

sysemu_supported = 1;
- printk("OK\n");
+ printf("OK\n");
set_using_sysemu(!force_sysemu_disabled);

- printk("Checking advanced syscall emulation patch for ptrace...");
+ printf("Checking advanced syscall emulation patch for ptrace...");
pid = start_ptraced_child(&stack);

if(ptrace(PTRACE_OLDSETOPTIONS, pid, 0,
@@ -246,7 +246,7 @@ static void __init check_sysemu(void)
goto fail_stopped;

sysemu_supported = 2;
- printk("OK\n");
+ printf("OK\n");

if ( !force_sysemu_disabled )
set_using_sysemu(sysemu_supported);
@@ -255,7 +255,7 @@ static void __init check_sysemu(void)
fail:
stop_ptraced_child(pid, stack, 1, 0);
fail_stopped:
- printk("missing\n");
+ printf("missing\n");
}

static void __init check_ptrace(void)
@@ -263,7 +263,7 @@ static void __init check_ptrace(void)
void *stack;
int pid, syscall, n, status;

- printk("Checking that ptrace can change system call numbers...");
+ printf("Checking that ptrace can change system call numbers...");
pid = start_ptraced_child(&stack);

if(ptrace(PTRACE_OLDSETOPTIONS, pid, 0, (void *)PTRACE_O_TRACESYSGOOD) < 0)
@@ -292,7 +292,7 @@ static void __init check_ptrace(void)
}
}
stop_ptraced_child(pid, stack, 0, 1);
- printk("OK\n");
+ printf("OK\n");
check_sysemu();
}

@@ -472,6 +472,8 @@ int can_do_skas(void)

int have_devanon = 0;

+/* Runs on boot kernel stack - already safe to use printk. */
+
void check_devanon(void)
{
int fd;
-
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