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Subject[PATCH 2/4] x86/dumpstack: remove kernel text addresses from stack dump
Date
Printing kernel text addresses in stack dumps is of questionable value,
especially now that address randomization is becoming common.

It can be a security issue because it leaks kernel addresses. It also
affects the usefulness of the stack dump. Linus says:

"I actually spend time cleaning up commit messages in logs, because
useless data that isn't actually information (random hex numbers) is
actively detrimental.

It makes commit logs less legible.

It also makes it harder to parse dumps.

It's not useful. That makes it actively bad.

I probably look at more oops reports than most people. I have not
found the hex numbers useful for the last five years, because they are
just randomized crap.

The stack content thing just makes code scroll off the screen etc, for
example."

The only real downside to removing these addresses is that they can be
used to disambiguate duplicate symbol names. However such cases are
rare, and the context of the stack dump should be enough to be able to
figure it out.

There's now a 'faddr2line' script which can be used to convert a
function address to a file name and line:

$ ./scripts/faddr2line ~/k/vmlinux write_sysrq_trigger+0x51/0x60
write_sysrq_trigger+0x51/0x60:
write_sysrq_trigger at drivers/tty/sysrq.c:1098

Or gdb can be used:

$ echo "list *write_sysrq_trigger+0x51" |gdb ~/k/vmlinux |grep "is in"
(gdb) 0xffffffff815b5d83 is in driver_probe_device (/home/jpoimboe/git/linux/drivers/base/dd.c:378).

(But note that when there are duplicate symbol names, gdb will only show
the first symbol it finds. faddr2line is recommended over gdb because
it handles duplicates and it also does function size checking.)

Here's an example of what a stack dump looks like after this change:

BUG: unable to handle kernel NULL pointer dereference at (null)
IP: sysrq_handle_crash+0x45/0x80
PGD 36bfa067 [ 29.650644] PUD 7aca3067
50970]
Oops: 0002 [#1] PREEMPT SMP
Modules linked in: ...
CPU: 1 PID: 786 Comm: bash Tainted: G E 4.9.0-rc1+ #1
Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.9.1-1.fc24 04/01/2014
task: ffff880078582a40 task.stack: ffffc90000ba8000
RIP: 0010:sysrq_handle_crash+0x45/0x80
RSP: 0018:ffffc90000babdc8 EFLAGS: 00010296
RAX: ffff880078582a40 RBX: 0000000000000063 RCX: 0000000000000001
RDX: 0000000000000001 RSI: 0000000000000000 RDI: 0000000000000292
RBP: ffffc90000babdc8 R08: 0000000b31866061 R09: 0000000000000000
R10: 0000000000000001 R11: 0000000000000000 R12: 0000000000000000
R13: 0000000000000007 R14: ffffffff81ee8680 R15: 0000000000000000
FS: 00007ffb43869700(0000) GS:ffff88007d400000(0000) knlGS:0000000000000000
CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 0000000000000000 CR3: 000000007a3e9000 CR4: 00000000001406e0
Stack:
ffffc90000babe00 ffffffff81572d08 ffffffff81572bd5 0000000000000002
0000000000000000 ffff880079606600 00007ffb4386e000 ffffc90000babe20
ffffffff81573201 ffff880036a3fd00 fffffffffffffffb ffffc90000babe40
Call Trace:
__handle_sysrq+0x138/0x220
? __handle_sysrq+0x5/0x220
write_sysrq_trigger+0x51/0x60
proc_reg_write+0x42/0x70
__vfs_write+0x37/0x140
? preempt_count_sub+0xa1/0x100
? __sb_start_write+0xf5/0x210
? vfs_write+0x183/0x1a0
vfs_write+0xb8/0x1a0
SyS_write+0x58/0xc0
entry_SYSCALL_64_fastpath+0x1f/0xc2
RIP: 0033:0x7ffb42f55940
RSP: 002b:00007ffd33bb6b18 EFLAGS: 00000246 ORIG_RAX: 0000000000000001
RAX: ffffffffffffffda RBX: 0000000000000046 RCX: 00007ffb42f55940
RDX: 0000000000000002 RSI: 00007ffb4386e000 RDI: 0000000000000001
RBP: 0000000000000011 R08: 00007ffb4321ea40 R09: 00007ffb43869700
R10: 00007ffb43869700 R11: 0000000000000246 R12: 0000000000778a10
R13: 00007ffd33bb5c00 R14: 0000000000000007 R15: 0000000000000010
Code: 34 e8 d0 34 bc ff 48 c7 c2 3b 2b 57 81 be 01 00 00 00 48 c7 c7 e0 dd e5 81 e8 a8 55 ba ff c7 05 0e 3f de 00 01 00 00 00 0f ae f8 <c6> 04 25 00 00 00 00 01 5d c3 e8 4c 49 bc ff 84 c0 75 c3 48 c7
RIP: sysrq_handle_crash+0x45/0x80 RSP: ffffc90000babdc8
CR2: 0000000000000000

Suggested-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Josh Poimboeuf <jpoimboe@redhat.com>
---
arch/x86/include/asm/kdebug.h | 1 -
arch/x86/kernel/dumpstack.c | 18 ++++--------------
arch/x86/kernel/process_32.c | 7 +++----
arch/x86/kernel/process_64.c | 6 +++---
arch/x86/mm/fault.c | 3 +--
arch/x86/platform/uv/uv_nmi.c | 4 ++--
6 files changed, 13 insertions(+), 26 deletions(-)

diff --git a/arch/x86/include/asm/kdebug.h b/arch/x86/include/asm/kdebug.h
index d318811..29a594a 100644
--- a/arch/x86/include/asm/kdebug.h
+++ b/arch/x86/include/asm/kdebug.h
@@ -21,7 +21,6 @@ enum die_val {
DIE_NMIUNKNOWN,
};

-extern void printk_address(unsigned long address);
extern void die(const char *, struct pt_regs *,long);
extern int __must_check __die(const char *, struct pt_regs *, long);
extern void show_stack_regs(struct pt_regs *regs);
diff --git a/arch/x86/kernel/dumpstack.c b/arch/x86/kernel/dumpstack.c
index 64281a1..f967652 100644
--- a/arch/x86/kernel/dumpstack.c
+++ b/arch/x86/kernel/dumpstack.c
@@ -46,14 +46,7 @@ static void printk_stack_address(unsigned long address, int reliable,
char *log_lvl)
{
touch_nmi_watchdog();
- printk("%s [<%p>] %s%pB\n",
- log_lvl, (void *)address, reliable ? "" : "? ",
- (void *)address);
-}
-
-void printk_address(unsigned long address)
-{
- pr_cont(" [<%p>] %pS\n", (void *)address, (void *)address);
+ printk("%s %s%pB\n", log_lvl, reliable ? "" : "? ", (void *)address);
}

void show_trace_log_lvl(struct task_struct *task, struct pt_regs *regs,
@@ -275,14 +268,11 @@ int __die(const char *str, struct pt_regs *regs, long err)
sp = kernel_stack_pointer(regs);
savesegment(ss, ss);
}
- printk(KERN_EMERG "EIP: [<%08lx>] ", regs->ip);
- print_symbol("%s", regs->ip);
- printk(" SS:ESP %04x:%08lx\n", ss, sp);
+ printk(KERN_EMERG "EIP: %pS SS:ESP: %04x:%08lx\n",
+ (void *)regs->ip, ss, sp);
#else
/* Executive summary in case the oops scrolled away */
- printk(KERN_ALERT "RIP ");
- printk_address(regs->ip);
- printk(" RSP <%016lx>\n", regs->sp);
+ printk(KERN_ALERT "RIP: %pS RSP: %016lx\n", (void *)regs->ip, regs->sp);
#endif
return 0;
}
diff --git a/arch/x86/kernel/process_32.c b/arch/x86/kernel/process_32.c
index 7dc8c9c..f854404 100644
--- a/arch/x86/kernel/process_32.c
+++ b/arch/x86/kernel/process_32.c
@@ -72,10 +72,9 @@ void __show_regs(struct pt_regs *regs, int all)
savesegment(gs, gs);
}

- printk(KERN_DEFAULT "EIP: %04x:[<%08lx>] EFLAGS: %08lx CPU: %d\n",
- (u16)regs->cs, regs->ip, regs->flags,
- smp_processor_id());
- print_symbol("EIP is at %s\n", regs->ip);
+ printk(KERN_DEFAULT "EIP: %pS\n", (void *)regs->ip);
+ printk(KERN_DEFAULT "EFLAGS: %08lx CPU: %d\n", regs->flags,
+ smp_processor_id());

printk(KERN_DEFAULT "EAX: %08lx EBX: %08lx ECX: %08lx EDX: %08lx\n",
regs->ax, regs->bx, regs->cx, regs->dx);
diff --git a/arch/x86/kernel/process_64.c b/arch/x86/kernel/process_64.c
index 4481e72..6c1b43e 100644
--- a/arch/x86/kernel/process_64.c
+++ b/arch/x86/kernel/process_64.c
@@ -61,10 +61,10 @@ void __show_regs(struct pt_regs *regs, int all)
unsigned int fsindex, gsindex;
unsigned int ds, cs, es;

- printk(KERN_DEFAULT "RIP: %04lx:[<%016lx>] %pS\n", regs->cs & 0xffff,
- regs->ip, (void *)regs->ip);
+ printk(KERN_DEFAULT "RIP: %04lx:%pS\n", regs->cs & 0xffff,
+ (void *)regs->ip);
printk(KERN_DEFAULT "RSP: %04lx:%016lx EFLAGS: %08lx", regs->ss,
- regs->sp, regs->flags);
+ regs->sp, regs->flags);
if (regs->orig_ax != -1)
pr_cont(" ORIG_RAX: %016lx\n", regs->orig_ax);
else
diff --git a/arch/x86/mm/fault.c b/arch/x86/mm/fault.c
index 9f72ca3..17c55a5 100644
--- a/arch/x86/mm/fault.c
+++ b/arch/x86/mm/fault.c
@@ -679,8 +679,7 @@ show_fault_oops(struct pt_regs *regs, unsigned long error_code,
printk(KERN_CONT "paging request");

printk(KERN_CONT " at %p\n", (void *) address);
- printk(KERN_ALERT "IP:");
- printk_address(regs->ip);
+ printk(KERN_ALERT "IP: %pS\n", (void *)regs->ip);

dump_pagetable(address);
}
diff --git a/arch/x86/platform/uv/uv_nmi.c b/arch/x86/platform/uv/uv_nmi.c
index cd5173a..8410e7d 100644
--- a/arch/x86/platform/uv/uv_nmi.c
+++ b/arch/x86/platform/uv/uv_nmi.c
@@ -387,8 +387,8 @@ static void uv_nmi_dump_cpu_ip_hdr(void)
/* Dump Instruction Pointer info */
static void uv_nmi_dump_cpu_ip(int cpu, struct pt_regs *regs)
{
- pr_info("UV: %4d %6d %-32.32s ", cpu, current->pid, current->comm);
- printk_address(regs->ip);
+ pr_info("UV: %4d %6d %-32.32s %pS",
+ cpu, current->pid, current->comm, (void *)regs->ip);
}

/*
--
2.7.4
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