Date: Sun, 8 May 2022 09:54:44 -0400 From: Rich Felker <dalias@...c.org> To: Markus Wichmann <nullplan@....net> Cc: musl@...ts.openwall.com, Pablo Galindo Salgado <pablogsal@...il.com> Subject: Re: Why the entries in the dynamic section are not always relocated? On Sun, May 08, 2022 at 01:39:10PM +0200, Markus Wichmann wrote: > On Sun, May 08, 2022 at 08:48:29AM +0100, Pablo Galindo Salgado wrote: > > Why is this happening? > > The easy question first: This is happening because glibc finds some > value in writing the actual addresses into the dynamic section, and musl > does not. All of the addresses given in the dynamic section must > necessarily be offsets into the library itself (rather, the run-time map > of the library), so anyone who knows the base address of the library can > interpret these values, anyway. That's basically it. musl does not do this mainly because it's not possible in general -- on some archs _DYNAMIC is in read-only memory -- and we generally avoid arch-specific behavior in the dynamic linker. The only part of _DYNAMIC we modify, on archs where it's allowed, is DT_DEBUG, because that's a (nasty, should be replaced) interface with debuggers to let them find things. > See, you are accessing an implementation detail here. I am not aware of > any documentation of dl_iterate_phdr() which says whether the dynamic > section is relocated or not. Which leads directly to: It's not so much in the scope of dl_iterate_phdr, but in the runtime contents of ELF data structures. There are specs on *some* of that, but they are not among the list of standards musl purports to conform to (and for example some things like handling of RPATH/RUNPATH intentionally differ from legacy behaviors here). > > How can one programmatically know when the linker is > > going to place here offsets or full > > relocated addresses? > > In general, you cannot. You could reconstruct the length of the library > mapping from the LOAD headers, then heuristically assume that any value > below that is an offset, and any value above it probably a pointer. > Doesn't help you far, though, since you also need the base address. > Though I suppose you could assume that the start of the page the PHDRs > start on is likely the base of the library mapping. > > Also, the heuristic will fail for libraries mapped to a low address. In > theory, all address space after the zero page is fair game, right? But > libraries can take more space than that. > > And God help you if you ever run into an FDPIC architecture. > > It appears to me that whatever you are trying to do is not possible > portibly on Linux at this time. Could you fill us in? Indeed, this is probably either an XY problem with a simple portable way to achieve whatever the underlying goal is, or a glorious hack that's making a lot more assumptions about implementation internals and not something you'd be able to rely on continuing to work in the future, even if you got it working. Rich
Powered by blists - more mailing lists
Confused about mailing lists and their use? Read about mailing lists on Wikipedia and check out these guidelines on proper formatting of your messages.