Introduce new relocation for landing pad

[kito-cheng]

The R_RISCV_LPAD relocation can be used for PLT entry generation and also for
linker relaxation. Additionally, we defined a new mapping symbol type to help
users understand the function signature for the corresponding function.

The addend value is the label value, and it will point to the mapping symbol
placed at the beginning of the function.

e.g.

foo:         # void foo(void)
$sFvvE:
    lpad 123 # R_RISCV_LPAD $sFvvE + 123

We propose two linker relaxations for the landing pad. The first is removing
the entire landing pad, which can be used when symbols have local visibility,
and the address is not taken by any other reference. The second is a landing
pad scheme conversion, designed for backward compatibility (or as a workaround)
for legacy programs that may use functions without declarations.

---

NOTE: This is based on #434

[kito-cheng]

cc @deepak0414 @ved-rivos @mylai-mtk

[aswaterman]

I think this reloc also provides enough info for the linker to fix up direct jumps to skip over landing pads when the landing pad was not relaxed.

Consider I have call foo, and the linker sees that the target of that call still has an R_RISCV_LPAD reloc (i.e. not relaxed). The linker should be able to adjust call foo to call foo+4. This should work whether the target is the PLT or the real function.

[deepak0414]

Q: what happens in case of dynamic linking ?

[kito-cheng]

I gave it some more thought, and this can actually be applied beyond just static linking. However, dependent shared libraries won’t automatically convert along with it. I’ve removed that limitation and added a NOTE to explain the situation.

[rui314]

I think we can explain the behavior more clearly if we avoid mentioning symbol visibility. The only important thing here is whether or not a symbol is visible to other ELF modules, i.e. whether or not the symbol is in the dynamic symbol table. Symbol visibility is just one way to control it.

[kito-cheng]

Good suggestion, applied 02546de :)

[rui314]

What is the "label value of the landing pad?

[rui314]

How should we find a function symbol for a given R_RISCV_LPAD relocation? Should we just look for a function symbol at the same location as the relocation refers to?

[kito-cheng]

Right now, this design requires scanning through the symbol table and relocation table once to figure out which symbols have landing pads. I had previously thought about creating a new section to handle this, but I realized that when dealing with linker relaxation, the best way is still to use relocations to mark them. This approach also avoids introducing a new section format.

[mylai-mtk]

Right now, this design requires scanning through the symbol table and relocation table once to figure out which symbols have landing pads.

Does this mean that O(R + F) time complexity and O(R) space complexity, where R is number of LPAD relocations and F is number of global function symbols in the same section, to build a map from function symbols to lpad labels is expected? This map is needed to synthesize PLT.

Note: The complexities come from the following algorithm:

HashMap<Address, LabelLabel> LpadRelocMap;
HashMap<FunctionSymbol, LpadLabel> LpadLabelMap;
for (auto R: LpadRelocations)
  LpadRelocMap.insert(R.Address, R.addend);

for (auto S: FunctionSymbols)
  if (S.Address in LpadRelocMap) lpadLabelMap.insert(S, LpadRelocMap[S.Address]);

[ved-rivos]

what would be the cases where a label may be computed incorrectly.

[kito-cheng]

Some legacy programs don’t properly declare function prototypes before calling them. In these cases, the compiler will infer a function prototype based on the language standards, but it often ends up being incorrect. One common example is dhrystone[1]. In most versions you find online, Func_2 isn’t declared before it’s called, so the compiler will assume the prototype is int Func_2(char*, char*), but the correct prototype is actually void Func_2(char[31], char[31]).

[1] https://github.com/sifive/benchmark-dhrystone/blob/master/dhry_1.c#L164

Another common potential issue in C is with qsort. Function pointers can be compatible but not perfectly match the expected type. For example, here’s how qsort is declared:

void qsort(void* ptr, size_t count, size_t size, int (*comp)(const void*, const void*));

But in practice, you can pass in a compatible, but not exactly matching, type for the comparison function, and it works in most cases:

#include <stdlib.h>

int compare(int *a, int *b)  // The signature isn’t int (*)(const void*, const void*)
{
    return *(int *)a - *(int *)b;
}

void foo(int *x, size_t count, size_t size)
{
    qsort(x, count, size, compare);  // But in practice, this works fine
}

[mylai-mtk]

But how is the linker expected to know the incorrectness so it can perform this relaxation?

The Zicfilp mechanism is employed when issuing an indirect call through function pointers, and when calling functions through PLT:

In the first case (indirect calls through pointers), to know that an lpad insn needs to be relaxed to lpad 0 due to the original label being incorrect, the linker would need to know where the pointer points to, so the caller's label (the "correct" one) can be checked against the callee's label (the lpad insn). I'm not sure if this is the scenario you're targeting, but if it is, I think this (knowing where the pointer points to, or knowing where the call would come from) is an expectation too high for linkers. Besides, in this scenario, I would also wonder how linkers are expected to retrieve the caller's label (the "correct" one) so it can make the comparison?

In the second case (calls through PLT), the indirect call happens in the PLT, which is generated by linkers. The label which linkers use to generate PLT would come from the addend of the LPAD relocation, which should contain the same label as the one in its referenced lpad insn, so there would be no chance of mismatch and thus incorrectness identifiable by linkers.

The above is my guess and understanding of the intended usage of this relaxation. If we're not on the same page, please do let me know.

[kito-cheng]

Linker never know (or not always know), and also that's not the right layer to analysis (or guess:P ), so I expect that relaxation should only enabled when user pass something like -z force-simple-landing-pad-scheme to linker.

[mylai-mtk]

I don't quite get the purpose of this mapping symbol: It looks like the only reference to these symbols come from the LPAD relocation, but for what? The LPAD relocation already have the 20-bit label stored in its addend, and its link to this %s mapping symbol provides only the additional information of the signature string, which is not needed (as of now) to link the object files.

[kito-cheng]

riscv/riscv-cfi#151 (comment)

It's kinda debugging propose only, so it safe to strip like all other mapping symbols

[mylai-mtk]

If the purpose is to display function signatures when disassembling, this mechanism seems a bit incomplete (?) I suppose since the relocation is a static one, it would not stay in the binary after static linking, thus if a user disassembles a linked ELF, it's still the label numbers instead of signatures that get displayed?

Update: Assuming it's relying on the mapping symbol having the same address as the lpad insn to associate an lpad insn to a function signature (so that the signature can be displayed when disassembling a linked binary), why do relocations refer to these symbols?

[mylai-mtk]

Q: Does the LPAD relocation and/or the $s<func-sig> mapping symbol serve the purpose of providing labels when generating PLT? If so, how is it expected to extract the label for a given function symbol from these new entities?

[rui314]

I don't think I fully understand this proposal, but from the linker's perspective, I believe we just need the following:

• A R_RISCV_LPAD relocation, whose r_offset is at each removable lpad instruction location, and r_sym refers to a function symbol
• If the function's address was not taken (i.e. the function symbol was not referenced by anything but R_RISCV_CALL or R_RISCV_CALL_PLT) and the function symbol is not in the dynamic symbol, the linker removes the lpad instruction.

I don't think we need a mapping symbol for the linker to remove a lpad instruction.

[kito-cheng]

@rui314

I don't think I fully understand this proposal, but from the linker's perspective, I believe we just need the following:

A R_RISCV_LPAD relocation, whose r_offset is at each removable lpad instruction location, and r_sym refers to a function symbol
If the function's address was not taken (i.e. the function symbol was not referenced by anything but R_RISCV_CALL or R_RISCV_CALL_PLT) and the function symbol is not in the dynamic symbol, the linker removes the lpad instruction.
I don't think we need a mapping symbol for the linker to remove a lpad instruction.

The design of the mapping symbol is meant to make debugging easier for users, so it’s actually optional. This was discussed in the CFI spec issue [1]. If we set aside that purpose, I also think it’s a better approach to have r_sym point to the function symbol.

As for using the addend to record the landing pad value, it helps with PLT generation. The linker can get the value by scanning the instruction at that address, but encoding it directly in the addend avoids needing to read the instruction during relocation.

[1] riscv/riscv-cfi#151 (comment)

[mylai-mtk]

Is this relocation only for the func-sig scheme? Based on its description, it looks like so, but the following LPAD relaxation that removes the lpad insn seems also applicable to the unlabeled scheme.

[kito-cheng]

That should also work for unlabeled scheme as well, let me think how to make it clearly.

[mylai-mtk]

@kito-cheng

As for using the addend to record the landing pad value, it helps with PLT generation. The linker can get the value by scanning the instruction at that address, but encoding it directly in the addend avoids needing to read the instruction during relocation.

I think this doesn't provide the lpad label we need for PLT generation? For a call target to be generated in the PLT, the target should reside in a shared library or somewhere that the linker cannot determine during static-time linking. When the target resides in a shared library, it does not have the LPAD relocation associated with it, since the relocation is a static relocation and should be consumed by the static linker that created the shared library?

[kito-cheng]

@mylai-mtk

I think this doesn't provide the lpad label we need for PLT generation? For a call target to be generated in the PLT, the target should reside in a shared library or somewhere that the linker cannot determine during static-time linking. When the target resides in a shared library, it does not have the LPAD relocation associated with it, since the relocation is a static relocation and should be consumed by the static linker that created the shared library?

Hmmmmmmmmmmmmmm, yeah, do you have any better idea that creating a new section to recording the label value for those undefined symbols?

[mylai-mtk]

@kito-cheng

Hmmmmmmmmmmmmmm, yeah, do you have any better idea that creating a new section to recording the label value for those undefined symbols?

Nope. After skimming through existing sections, I don't have ideas better than creating a new section.

My previous prototype of generating extra symbols for every function may work, but there's too many drawbacks with this approach, including bloated symbol table size and slowed down symbol resolution performance, so I would not recommend this approach seriously.