[Libunwind-devel] I don't get unw_set_reg

[Doug Moore]

I’m working on a program that tries to walk up the stack after an interrupt, 
record information about the stack, and resume as if the interrupt never 
happened.  Occasionally, unw_step fails, and, because we have some local 
expertise in x86-64 stack unwinding, we try to figure out how to complete the 
step ourselves.  After the try, we’d like to use unw_set_reg to update the 
cursor with the new values we’ve determined and resume the use of libunwind.

The program under test - not the one I’m working on, but the one that we are 
analyzing with it - segfaults if I dare use unw_set_reg on the libunwind 
cursor, even though I never try to use unw_resume to use the cursor to affect 
the program.

I can see that unw_step affects the cursor->loc member of the dwarf cursor, but 
does not go to the underlying context to change anything, while unw_get_reg and 
unw_set_reg do go to the context, so maybe I’ve looking for an accessible way 
to access/modify the loc fields of the dwarf cursor.  However, the textbook 
example of writing a stack trace uses unw_step and unw_get_reg, so going to the 
underlying context must be okay(?).  How does unw_get_reg get the value that 
unw_step has modified, when it seems to be looking past that to the underlying 
context?

So, I’m confused about things.  How can I expect unw_get_reg to work for stack 
unwinding, when unw_set_reg crashes the program?

Help appreciated.

Doug Moore