Hello all,
In a small exchange in fedora-kernel-list [1] Eric Sandeen has pointed
out a possible stack overflow... when CONFIG_DEBUG_STACKOVERFLOW is
enabled. (Though not limited to it)
Code path is simple: do_IRQ detects a a near stack overflow condition
and calls show_trace_log_lvl which, down the line uses __print_symbol
and sprint_symbol to print the call stack.
However, both __print_symbol + sprint_symbol are eating no-less then
128+223 bytes on static char arrays, which, given the fact that this
code path is actually generated by low stack warning (< 512 bytes),
might turn a minor (?) problem (low stack) into a full blown crash.
The patch itself is fairly simple and non-intrusive. [2]
Both functions allocate memory for their buffers - falling back to
minimal address display if memory allocation fails.
P.S. Can anyone please point me to the maintainer of kernel/syms? (I
rather not spam world + dog for such a minor patch)
--
Gilboa Davara <[email protected]>
[1]
http://www.mail-archive.com/[email protected]/msg00640.html
[2]. In theory, there's a second option: pre-allocating memory on a
per_cpu basis, however:
A. dump_trace/stack are usually called when something bad has happened -
reducing the need for performance optimizations.
B. per_cpu allocation will also require local_irq_disable/enable as both
functions are being called from multiple contexts. Too much hassle.
--- linux-2.6/kernel/kallsyms.orig 2007-09-15 11:46:54.000000000 +0300
+++ linux-2.6/kernel/kallsyms.c 2007-09-15 14:25:21.000000000 +0300
@@ -309,30 +309,62 @@ int lookup_symbol_attrs(unsigned long ad
/* Look up a kernel symbol and return it in a text buffer. */
int sprint_symbol(char *buffer, unsigned long address)
{
- char *modname;
- const char *name;
unsigned long offset, size;
- char namebuf[KSYM_NAME_LEN];
+ const char *name = NULL;
+ char *namebuf = NULL;
+ char *modname;
+ int ret;
+
+
+ /* Static buffer allocation.
+ Required in-order to reduce stack footprint on
+ do_IRQ/4KSTACK/i386 */
+ namebuf = kmalloc(KSYM_NAME_LEN, GFP_ATOMIC);
+ if (namebuf)
+ name = kallsyms_lookup(address, &size, &offset,
+ &modname, namebuf);
- name = kallsyms_lookup(address, &size, &offset, &modname, namebuf);
if (!name)
- return sprintf(buffer, "0x%lx", address);
+ ret = sprintf(buffer, "0x%lx", address);
+ else {
+ if (modname)
+ ret = sprintf(buffer, "%s+%#lx/%#lx [%s]",
+ name, offset, size, modname);
+ else
+ ret = sprintf(buffer, "%s+%#lx/%#lx",
+ name, offset, size);
+ }
- if (modname)
- return sprintf(buffer, "%s+%#lx/%#lx [%s]", name, offset,
- size, modname);
- else
- return sprintf(buffer, "%s+%#lx/%#lx", name, offset, size);
+ if (namebuf)
+ kfree(namebuf);
+
+ return ret;
}
/* Look up a kernel symbol and print it to the kernel messages. */
void __print_symbol(const char *fmt, unsigned long address)
{
- char buffer[KSYM_SYMBOL_LEN];
+ char *buffer = NULL;
- sprint_symbol(buffer, address);
- printk(fmt, buffer);
+ /* Static buffer allocation.
+ Required in-order to reduce stack footprint on
+ do_IRQ/4KSTACK/i386 */
+ buffer = kmalloc(KSYM_SYMBOL_LEN, GFP_ATOMIC);
+ if (buffer) {
+ sprint_symbol(buffer, address);
+ printk(fmt, buffer);
+ kfree(buffer);
+ } else {
+ /* Address + '0x' + NULL. */
+ char sbuffer[(BITS_PER_LONG / 4) + 3];
+
+ /* Fall-back mode.
+ Memory allocation failed.
+ Convert the address to string and display it. */
+ sprintf(sbuffer, "0x%lx", address);
+ printk(fmt, sbuffer);
+ }
}
/* To avoid using get_symbol_offset for every symbol, we carry prefix
along. */
-
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