On Tue, Sep 22, 2020 at 09:46:16PM -0400, Arvind Sankar wrote: > 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. For libffi, I think the proposed standard trampoline won't actually work, because not all ABIs have two scratch registers available to use as code_reg and data_reg. Eg i386 fastcall only has one, and register has zero scratch registers. I believe 32-bit ARM only has one scratch register as well. For i386 you'd need something that saves a register on the stack first, maybe like the below with a 16-byte trampoline and a 16-byte context structure that has the address of the code to jump to in the first dword: .balign 4096 trampoline_page: .rept 4096/16-1 0: endbr32 push %eax call __x86.get_pc_thunk.ax 1: jmp trampoline .balign 16 .endr .org trampoline_page + 4096 - 16 __x86.get_pc_thunk.ax: movl (%esp), %eax ret trampoline: subl $(1b-0b), %eax jmp *(table-trampoline_page)(%eax) .org trampoline_page + 4096 table:
