| Message ID | 20240405083539.374995-10-npiggin@gmail.com (mailing list archive) |
|---|---|
| State | New |
| Headers | show |
| Series | migration, powerpc improvements | expand |
On 05/04/2024 10.35, Nicholas Piggin wrote: > The backtrace handler terminates when it sees a NULL caller address, > but the powerpc stack setup does not keep such a NULL caller frame > at the start of the stack. > > This happens to work on pseries because the memory at 0 is mapped and > it contains 0 at the location of the return address pointer if it > were a stack frame. But this is fragile, and does not work with powernv > where address 0 contains firmware instructions. > > Use the existing dummy frame on stack as the NULL caller, and create a > new frame on stack for the entry code. > > Signed-off-by: Nicholas Piggin <npiggin@gmail.com> > --- > powerpc/cstart64.S | 15 +++++++++++++++ > 1 file changed, 15 insertions(+) Reviewed-by: Thomas Huth <thuth@redhat.com>
diff --git a/powerpc/cstart64.S b/powerpc/cstart64.S index e18ae9a22..80baabe8f 100644 --- a/powerpc/cstart64.S +++ b/powerpc/cstart64.S @@ -46,6 +46,21 @@ start: add r1, r1, r31 add r2, r2, r31 + /* Zero backpointers in initial stack frame so backtrace() stops */ + li r0,0 + std r0,0(r1) + std r0,16(r1) + + /* + * Create entry frame of 64-bytes, same as the initial frame. A callee + * may use the caller frame to store LR, and backtrace() termination + * looks for return address == NULL, so the initial stack frame can't + * be used to call C or else it could overwrite the zeroed LR save slot + * and break backtrace termination. This frame would be unnecessary if + * backtrace looked for a zeroed frame address. + */ + stdu r1,-64(r1) + /* save DTB pointer */ std r3, 56(r1)
The backtrace handler terminates when it sees a NULL caller address, but the powerpc stack setup does not keep such a NULL caller frame at the start of the stack. This happens to work on pseries because the memory at 0 is mapped and it contains 0 at the location of the return address pointer if it were a stack frame. But this is fragile, and does not work with powernv where address 0 contains firmware instructions. Use the existing dummy frame on stack as the NULL caller, and create a new frame on stack for the entry code. Signed-off-by: Nicholas Piggin <npiggin@gmail.com> --- powerpc/cstart64.S | 15 +++++++++++++++ 1 file changed, 15 insertions(+)