Messages in this thread | | | From | Stijn Volckaert <> | Subject | Re: vdso feature requests from the Go people | Date | Sun, 15 Jun 2014 11:39:14 +0200 |
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Andy Lutomirski schreef op 13/06/2014 7:23: > On Thu, Jun 12, 2014 at 10:15 PM, H. Peter Anvin <hpa@zytor.com> wrote: >> On 06/12/2014 09:36 PM, Andy Lutomirski wrote: >>> >>> If we were to implement both, maybe we'd actually want to provide >>> something like: >>> >>> struct vdso_entry { >>> unsigned long vdso_entry_struct_size; /* so we can add fields later on */ >>> void *func; >>> unsigned int max_stack; /* zero if not known */ >>> }; >>> >> >> How different is this really from the .dynsym table -- other than that >> you have to find the latter via the dynamic segment? >> >> The hash part is just to speed up lookup... > > As far as I know, there's no reliable way to just read the dynsym > table -- the thing doesn't have a specified length, which is what > broke Go in the first place. > > Parsing the ELF dynamic tables is kind of annoyingly complicated, and > understanding the format is a real PITA -- the documentation I've been > able to find is outright terrible. > > --Andy >
Why does it need a specified length? It's pretty easy to find, the entries have a fixed length and the last entry in the table is NULLed out (i.e. the d_tag is DT_NULL).
Just look for the PT_DYNAMIC segment in the program headers and iterate over the fixed length Elf_Dyn entries to find what you're looking for.
As for ELF documentation, I've never had any problems understanding the official specification: http://refspecs.linuxbase.org/elf/elf.pdf
--Stijn
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