Hi, CCing the elfutils-devel list where this issue (of missing DT_SYMTAB_COUNT or DT_SYMTABSZ) does come up occasionally. Which are needed if you only have the dynamic segment, and not the section headers, to enumerate all symbols.
On Mon, Aug 08, 2022 at 05:29:19PM -0700, Cary Coutant wrote: > > We recently had a case where dropping DT_HASH from the generic > > glibc binaries broke an ELF consumer. > > > > For the "real world" details see: > > https://sourceware.org/pipermail/libc-alpha/2022-August/141302.html > > > > I was surprised to see that DT_HASH was "mandatory" in the existing > > published standard. > > > > Why is it mandatory and not optional? > > > > Should we and could we make it optional? > > This is an odd situation. > > DT_HASH is mandatory because the "nchain" field in the hash table is > the only way to know the number of symbols in DT_SYMTAB (unless you > still have a section table and look for the .dynsym section, which is > not something we want to require). But when the psABI has effectively > replaced DT_HASH with something newer, it makes sense for that > requirement to transfer to the newer table. I suppose the gABI could > say something like "mandatory unless the psABI provides for a > replacement in some form," but that doesn't feel right to me. In this > case, I think it's simply up to the psABI to override the gABI and say > that DT_HASH is optional if DT_GNU_HASH is present. Unfortunately, > that means that generic tools that know nothing of ELFOSABI_GNU or > ELFOSABI_LINUX would be unable to process DT_SYMTAB if they can't find > DT_HASH. > > I'd probably have preferred to have a mandatory DT_SYMTAB_COUNT or > DT_SYMTABSZ entry in the dynamic table, which would have enabled us to > make the hash table (in any form) optional. But the DT_HASH > requirement has been with us since the beginning of time. Do you know the reason for not having a DT_SYMTAB_COUNT or DT_SYMTABSZ? It is not intuitive that one can find the number of entries through the hash table. I don't think DT_GNU_HASH can simply be made optional and replace DT_HASH without adding something like DT_SYMTAB_COUNT or DT_SYMTABSZ because the gnu hashtable doesn't come with a simple symbol count. To find the symbol count you have to go through the whole gnu hash table to count the number of entries (which is a non-trivial amount of work). Cheers, Mark
