[PATCH] D69093: [llvm-objcopy] --add-symbol: fix crash if SHT_SYMTAB does not exist
Jordan Rupprecht via Phabricator via llvm-commits
llvm-commits at lists.llvm.org
Mon Oct 21 10:36:53 PDT 2019
rupprecht added inline comments.
================
Comment at: llvm/tools/llvm-objcopy/ELF/Object.cpp:1550-1552
+ // Prefer .strtab to .shstrtab.
+ if (Obj.SectionNames != &Sec)
+ break;
----------------
MaskRay wrote:
> 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.
Yes, exactly. Repro:
```
$ cat strtab.yaml
--- !ELF
FileHeader:
Class: ELFCLASS64
Data: ELFDATA2LSB
Type: ET_REL
Machine: EM_X86_64
Sections:
- Name: .strtab
Type: SHT_STRTAB
Flags: [ SHF_ALLOC ]
$ yaml2obj strtab.yaml > strtab.o
$ llvm-objcopy --add-symbol=abs1=1 strtab.o strtab2.o
$ llvm-readobj --sections strtab2.o
...
Section {
Index: 1
Name: .strtab (11)
Type: SHT_STRTAB (0x3)
Flags [ (0x2)
SHF_ALLOC (0x2)
]
}
Section {
Index: 2
Name: .shstrtab (1)
Type: SHT_STRTAB (0x3)
Flags [ (0x0)
]
}
Section {
Index: 3
Name: .symtab (19)
Type: SHT_SYMTAB (0x2)
Flags [ (0x0)
]
Link: 2 # <-- .shstrtab, not .strtab
}
...
```
Running the same example through a recent GNU objcopy, it looks like it creates a second `.strtab` without the `SHF_ALLOC` bit and links to that:
```
$ llvm-objcopy --add-symbol=abs1=1 strtab.o strtab2.o
$ llvm-readobj --sections strtab2.o
Section {
Index: 1
Name: .strtab (9)
Type: SHT_STRTAB (0x3)
Flags [ (0x2)
SHF_ALLOC (0x2)
]
}
Section {
Index: 2
Name: .symtab (1)
Type: SHT_SYMTAB (0x2)
Flags [ (0x0)
]
Link: 3 # <-- non-SHF_ALLOC .strtab below, not the SHF_ALLOC .strtab above
}
Section {
Index: 3
Name: .strtab (9)
Type: SHT_STRTAB (0x3)
Flags [ (0x0)
]
}
Section {
Index: 4
Name: .shstrtab (17)
Type: SHT_STRTAB (0x3)
Flags [ (0x0)
]
}
```
This behavior seems reasonable to me vs re-using the .shstrtab for strings that aren't section headers.
> To confirm, I think ... && !(Sec.Flags & SHF_ALLOC) is correct here.
>
BTW, I don't find it particularly constructive to simply state a position without any rationale.
Repository:
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https://reviews.llvm.org/D69093/new/
https://reviews.llvm.org/D69093
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