On Sat, Jul 4, 2020 at 6:29 AM Valentin Vidić <vvidic@valentin-vidic.from.hr> wrote:

On Sat, Jul 04, 2020 at 06:11:19AM -0400, William Tambe wrote:

...

I am seeing an issue in our Linux port where printk() format %pS will print a symbol as: kernel_init+0x120/0x120 where the offset within the function 0x120, is the same as the function size 0x120; in fact, the largest offset within the function should be 0x11f.

When printing above symbol+offset in gdb, the correct symbol is displayed as follow:

(gdb) p (void*)(kernel_init+0x120) $1 = (void *) 0x589a28 <ret_for_syscall>

What could be the cause of printk() format %pS printing an incorrect symbol.

Where can I find in the Linux source code, the function handling the format %pS ?

Check lib/vsprintf.c

How or which function within that file translates an address to the string symbol+offset for the format %pS ?

-- Valentin

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On Sat, Jul 4, 2020 at 1:15 PM Valentin Vidić <vvidic@valentin-vidic.from.hr> wrote:

On Sat, Jul 04, 2020 at 12:04:59PM -0400, William Tambe wrote:

...

How or which function within that file translates an address to the string symbol+offset for the format %pS ?

%pS seems to end up in here:

Thanks, the issue I am having is due to sprint_backtrace() calling __sprint_symbol() with its argument symbol_offset == -1. Despite the comment above its definition, it is hard to understand why sprint_backtrace() calls __sprint_symbol() that way; in our port it results in printing incorrect symbols. As a workaround, we have made sprint_backtrace() to be the same as sprint_symbol().

static int __sprint_symbol(char *buffer, unsigned long address, int symbol_offset, int add_offset) { char *modname; const char *name; unsigned long offset, size; int len;

address += symbol_offset; name = kallsyms_lookup(address, &size, &offset, &modname, buffer); if (!name) return sprintf(buffer, "0x%lx", address - symbol_offset);

if (name != buffer) strcpy(buffer, name); len = strlen(buffer); offset -= symbol_offset;

if (add_offset) len += sprintf(buffer + len, "+%#lx/%#lx", offset, size);

if (modname) len += sprintf(buffer + len, " [%s]", modname);

return len; }

-- Valentin

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On Tue, Jul 07, 2020 at 08:39:27PM +0200, Valentin Vidić wrote:

From what I understand print_backtrace() tries to handle the case when call is the last instruction in a function:

func1: ... ... ... call noret_func3() func2: ... ... ...

Return value on the stack points to the next instruction after the call. But in this case a new function already starts on that address so they add -1 to make the address point back to func1.

Not sure what goes wrong in your case, could you share an example and more info on the port?

This could go wrong if the return address on the stack points to the first instruction of ret_for_syscall: kernel_init: ... ... ... ret_for_syscall: ... <-- return address ... ... In this case kernel_init would be printed instead. Not sure how this address would end up on the stack unless ret_for_syscall is placed directly on the stack for some reason? -- Valentin