On Tue, 13 Feb 2018, Chet Ramey wrote: > > Note that a bash binary compiled with PIE works fine for normal usage > > with a current Linux kernel. Apparently it was causing troubles with > > older Linux kernels, see > > https://bugs.launchpad.net/ubuntu/+source/linux/+bug/1518483 > > > > But with current kernels it works fine. The problem only happens > > under qemu-user. > > Wouldn't that imply that the problem has something to do with > qemu-user and how it interacts with sbrk?
Possibly, I opened a ticket there as well to see what they think of this: https://bugs.launchpad.net/qemu/+bug/1749393 > > From what I understood the memory layout of a PIE-compiled binary is > > different but sbrk() still works the same. Maybe some assumption that you > > are doing about the memory layout are not holding true however. > > The only assumption bash makes is that sbrk() extends the break and that > the pointer it returns marks the beginning of contiguously addressable > storage of the requested size. > > > While trying to learn more about this I found this URL > > https://gist.github.com/CMCDragonkai/10ab53654b2aa6ce55c11cfc5b2432a4 > > The applicable part of this is that sbrk() extends the heap. One thing that I saw in that document is "An interesting fact is that if you produce a position independent executable, the starting address instead changes to 0x0". Isn't it possible that sbrk() returns that pointer to you and you treat it as being an error instead of a valid address? > The thing about the error message in the ubuntu bug report is that it's > literally the first call to xmalloc bash makes at startup: the call to > savestring when saving the default locale. You can tell because it reports > 0 bytes as having been allocated. sbrk() fails immediately. In my tests under qemu-user, I see this: bash: xmalloc: .././shell.c:1709: cannot allocate 10 bytes (0 bytes allocated) It's "savestring (shell_name)". Cheers, -- Raphaël Hertzog ◈ Debian Developer Support Debian LTS: https://www.freexian.com/services/debian-lts.html Learn to master Debian: https://debian-handbook.info/get/
