[PATCH] D69093: [llvm-objcopy] --add-symbol: fix crash if SHT_SYMTAB does not exist

[Fangrui Song via Phabricator via llvm-commits]

MaskRay marked an inline comment as done.
MaskRay added inline comments.


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Comment at: llvm/tools/llvm-objcopy/ELF/Object.cpp:1550-1552
+        // Prefer .strtab to .shstrtab.
+        if (Obj.SectionNames != &Sec)
+          break;
----------------
grimar wrote:
> grimar wrote:
> > MaskRay wrote:
> > > abrachet wrote:
> > > > rupprecht wrote:
> > > > > MaskRay wrote:
> > > > > > rupprecht wrote:
> > > > > > > Should `.dynstr` also be avoided? Is it better to just check that the name is literally `.strtab`?
> > > > > > .dynstr is avoided by `!(Sec.Flags & SHF_ALLOC)`. `.strtab` works as well but `Obj.SectionNames != &Sec` is more general. There are multiple ways to accomplish the samething. I just wanted to make the code less magical.
> > > > > From: http://www.sco.com/developers/gabi/latest/ch4.sheader.html#special_sections
> > > > > > .strtab
> > > > > > This section holds strings, most commonly the strings that represent the names associated with symbol table entries. If the file has a loadable segment that includes the symbol string table, the section's attributes will include the SHF_ALLOC bit; otherwise, that bit will be off.
> > > > > 
> > > > > So, `.strtab` can also be `SHF_ALLOC`.
> > > > For reference, we used to have a file in test/tools/llvm-objcopy/ELF/Inputs/ called alloc-symtab which had an SHF_ALLOC .strtab. It was removed in D65278. 
> > > To confirm, I think `... && !(Sec.Flags & SHF_ALLOC)` is correct here.
> > > For reference, we used to have a file in test/tools/llvm-objcopy/ELF/Inputs/ called alloc-symtab which had an SHF_ALLOC .strtab. It was removed in D65278.
> > 
> > Thanks for the reference. I completely forgot that saw an allocatable `.symtab` that time.
> > 
> > To clarify: in D65278 I've replaced a precompiled binary with a YAML description (with a SHF_ALLOC `.symtab`).
> > The logic of tests shouldn't have been affected, because tests wanted to see the SHF_ALLOC symbol table (but not SHF_ALLOC string table)
> > I believe.
> > 
> > If we need a test with a SHF_ALLOC `.strtab`, it should be possible to craft one with a yaml2obj too I think.
> > (I haven't check, but I think all that needed is to define a string table section and a `Flag` property which should override the default).
> > 
> > So, .strtab can also be SHF_ALLOC.
> 
> > To confirm, I think ... && !(Sec.Flags & SHF_ALLOC) is correct here.
> 
> To summarize and clarify this a bit:
> This part of code is called when there is no symbol table (`.symtab`).
> Jordan mentioned that `.strtab` can be allocatable in according to specification.
> 
> So if I understand correctly, the question is: is it possible that
> we have no symbol table, but have an allocatable `.strtab` in the object?
> 
> If it is, this code might either use the `.shstrtab` instead of existent `.strtab`,
> or perform an attemp to create a new string table (if there is no `.shstrtab` to reuse) it seems.
> Then it will create a `.symtab` using the string table either found or created instead of the existent allocatable '.strtab'.
> Does not sound correct perhaps?
> 
Both SHF_ALLOC .strtab and non-SHF_ALLOC .strtab are possible, though I don't think ld or objcopy could create SHF_ALLOC .strtab.

Generally objcopy should not alter SHF_ALLOC sections. The code skips both .dynstr (SHF_ALLOC) and SHF_ALLOC .strtab.  SHF_ALLOC .strtab is not tested, though.


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  https://reviews.llvm.org/D69093/new/

https://reviews.llvm.org/D69093