Date: Fri, 5 Mar 2021 10:07:32 -0500 From: Rich Felker <dalias@...c.org> To: Michael Forney <mforney@...rney.org> Cc: musl@...ts.openwall.com Subject: Re: ld-musl-* and empty .eh_frame On Thu, Mar 04, 2021 at 07:18:11PM -0800, Michael Forney wrote: > Hi, > > Érico noticed that cproc (my C compiler) produced executables that > musl's dynamic linker fails to load when passed as an argument: > > /lib/ld-musl-x86_64.so.1: ./t: Not a valid dynamic program > > However, running ./t directly works fine. It turns out that this > is because the executables have an empty .eh_frame section, which > causes musl to attempt an mmap with length 0 which fails with EINVAL. The section itself isn't the problem; rather the linker making a dedicated PROT_READ segment with no non-zero-length sections in it is. It really should have collapsed that out. (Also it would not happen without the separate-text option, which mcm disables because it makes lots of problems.) With that said, there's no good reason we should error out on this; it's syntactically and semantically valid just pointless for the linker to emit. I think adding if (!n) return p; at the top of mmap_fixed in dynlink.c fixes it. > This leaves me with a few questions: > > 1. Is it invalid for an ELF executable to have an empty .eh_frame > section? The only documentation I could find about it is , > which says that it must contain one or more CFI records, so 0 > would be invalid. > 2. Is it the compiler's responsibility to link with an object > containing a CIE terminator (like gcc's crtend.o) to prevent an > empty .eh_frame section? Sections are irrelevant to an executable file so it doesn't matter whatsoever. They're involved only in pre-link contracts and debugging. > 3. Is it a bug that GNU ld creates an empty .eh_frame by default, > even when none of the objects it is linking have one? It looks > like lld does not create an .eh_frame in this case. I don't think so. I think the bug is in the segment logic. > 4. Should musl's ld.so be able to handle such executables? The > kernel does not seem to have a problem with it, as well glibc's > ld.so with an executable I crafted with a 0-length .eh_frame > section. Yes. Rich
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