On Thu, Sep 17, 2020 at 10:36:02AM -0500, Madhavan T. Venkataraman wrote: > > > On 9/16/20 8:04 PM, Florian Weimer wrote: > > * madvenka: > > > >> Examples of trampolines > >> ======================= > >> > >> libffi (A Portable Foreign Function Interface Library): > >> > >> libffi allows a user to define functions with an arbitrary list of > >> arguments and return value through a feature called "Closures". > >> Closures use trampolines to jump to ABI handlers that handle calling > >> conventions and call a target function. libffi is used by a lot > >> of different applications. To name a few: > >> > >> - Python > >> - Java > >> - Javascript > >> - Ruby FFI > >> - Lisp > >> - Objective C > > > > libffi does not actually need this. It currently collocates > > trampolines and the data they need on the same page, but that's > > actually unecessary. It's possible to avoid doing this just by > > changing libffi, without any kernel changes. > > > > I think this has already been done for the iOS port. > > > > The trampoline table that has been implemented for the iOS port (MACH) > is based on PC-relative data referencing. That is, the code and data > are placed in adjacent pages so that the code can access the data using > an address relative to the current PC. > > This is an ISA feature that is not supported on all architectures. > > Now, if it is a performance feature, we can include some architectures > and exclude others. But this is a security feature. IMO, we cannot > exclude any architecture even if it is a legacy one as long as Linux > is running on the architecture. So, we need a solution that does > not assume any specific ISA feature. Which ISA does not support PIC objects? You mentioned i386 below, but i386 does support them, it just needs to copy the PC into a GPR first (see below). > > >> The code for trampoline X in the trampoline table is: > >> > >> load &code_table[X], code_reg > >> load (code_reg), code_reg > >> load &data_table[X], data_reg > >> load (data_reg), data_reg > >> jump code_reg > >> > >> The addresses &code_table[X] and &data_table[X] are baked into the > >> trampoline code. So, PC-relative data references are not needed. The user > >> can modify code_table[X] and data_table[X] dynamically. > > > > You can put this code into the libffi shared object and map it from > > there, just like the rest of the libffi code. To get more > > trampolines, you can map the page containing the trampolines multiple > > times, each instance preceded by a separate data page with the control > > information. > > > > If you put the code in the libffi shared object, how do you pass data to > the code at runtime? If the code we are talking about is a function, then > there is an ABI defined way to pass data to the function. But if the > code we are talking about is some arbitrary code such as a trampoline, > there is no ABI defined way to pass data to it except in a couple of > platforms such as HP PA-RISC that have support for function descriptors > in the ABI itself. > > As mentioned before, if the ISA supports PC-relative data references > (e.g., X86 64-bit platforms support RIP-relative data references) > then we can pass data to that code by placing the code and data in > adjacent pages. So, you can implement the trampoline table for X64. > i386 does not support it. > i386 just needs a tiny bit of code to copy the PC into a GPR first, i.e. the trampoline would be: call 1f 1: pop %data_reg movl (code_table + X - 1b)(%data_reg), %code_reg movl (data_table + X - 1b)(%data_reg), %data_reg jmp *(%code_reg) I do not understand the point about passing data at runtime. This trampoline is to achieve exactly that, no? Thanks.
