On Mon, 2 Mar 2020 at 18:02, Arvind Sankar <nivedita@xxxxxxxxxxxx> wrote: > > On Mon, Mar 02, 2020 at 05:57:04PM +0100, Ard Biesheuvel wrote: > > On Mon, 2 Mar 2020 at 17:54, Arvind Sankar <nivedita@xxxxxxxxxxxx> wrote: > > > > > > On Mon, Mar 02, 2020 at 08:49:17AM +0100, Ard Biesheuvel wrote: > > > > On Mon, 2 Mar 2020 at 00:04, Arvind Sankar <nivedita@xxxxxxxxxxxx> wrote: > > > ... > > > > > call 1f > > > > > -1: pop %ebp > > > > > - subl $1b, %ebp > > > > > +1: pop %ebx > > > > > + subl $1b, %ebx > > > ... > > > > > > > > > > + movl %ebx, %ebp // startup_32 for efi32_pe_stub_entry > > > > > > > > The code that follows efi32_pe_stub_entry still expects the runtime > > > > displacement in %ebp, so we'll need to pass that in another way here. > > > > > > > > > jmp efi32_pe_stub_entry > > > > > > Didn't follow -- what do you mean by runtime displacement? > > > > > > efi32_pe_stub_entry expects the runtime address of startup_32 to be in > > > %ebp, but with the changes for keeping the frame pointer in %ebp, I > > > changed the runtime address to be in %ebx instead. Hence I added that > > > movl %ebx, %ebp to put it in %ebp just before calling efi32_pe_stub_entry. > > > That should be fine, no? > > > > But how does that work with: > > > > SYM_INNER_LABEL(efi32_pe_stub_entry, SYM_L_LOCAL) > > movl %ecx, efi32_boot_args(%ebp) > > movl %edx, efi32_boot_args+4(%ebp) > > movb $0, efi_is64(%ebp) > > > > > > ? > > Why wouldn't it work? Before this change, efi32_pe_entry set %ebp to > startup_32 (via the call/pop/sub sequence), so efi32_pe_stub_entry was > entered with %ebp == startup_32. > > After this change, the call/pop/sub sequence puts startup_32 into %ebx, > and then I copy it into %ebp just before branching to efi32_pe_stub_entry. > So everything should continue to work the same way as before? Ah ok, so the sequence call 1f 1: pop %ebp subl $1b, %ebp happens to produce the value of startup_32, which is obvious now that I think of it, but it wasn't before.
