[sysvabi64] Document requirements for tools wrt BTI #282
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Add requirements for when a tool must generate a BTI instruction. This permits tools to elide BTI instructions when they can prove that no indirect branch to that location is possible from local information available to the tool. Static linkers are not allowed to assume that all direct branch targets have a BTI instruction. If a veneer is required then the static linker must generate additional BTI instructions if needed. A static linker is allowed to assume that a symbol that is exported to the dynamic symbol table has a BTI instruction. In practice this will permit compilers to remove BTI instructions from static functions that do not have their address taken and that address escapes the function. This matches the behavior of the GNU toolchain. Fixes ARM-software#196
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LLD patch to implement BTI aware thunks llvm/llvm-project#108989 |
It is possible to provide a static linker with branch targets using absolute symbols. For example using --defsym. The static cannot disassemble the location of these symbols so it isn't possible to check whether A BTI landing pad is necessary, nor is it always possible to place a BTI landing pad within range of the absolute symbol address. A user providing the static linker with function addresses using absolute symbols is responsible for putting BTI compatible landing pads at the start of these addresses.
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Add requirements for when a tool must generate a BTI instruction. This permits tools to elide BTI instructions when they can prove that no indirect branch to that location is possible from local information available to the tool.
Static linkers are not allowed to assume that all direct branch targets have a BTI instruction. If a veneer is required then the static linker must generate additional BTI instructions if needed.
A static linker is allowed to assume that a symbol that is exported to the dynamic symbol table has a BTI instruction.
In practice this will permit compilers to remove BTI instructions from static functions that do not have their address taken and that address escapes the function.
This matches the behavior of the GNU toolchain.
Fixes #196