[lldb-dev] Fixing cross platform mac to linux remote expression evaluation
Greg Clayton
gclayton at apple.com
Tue May 5 11:44:09 PDT 2015
You need to use mmap() because that is the only way to change the type of the memory to be executable. If you use malloc, you get read+write memory and can't change it to restrict the type. Allocate memory can request read, write and execute (any combo of the three).
> On May 4, 2015, at 2:54 PM, Robert Flack <flackr at gmail.com> wrote:
>
> Please let me know if I'm missing something, as far as I can see lldb-server does not have a Process object, only a NativeProcessLinux (which is a NativeProcessProtocol). This doesn't seem to have the context to run the thread plan to call malloc and allocate memory from lldb-server - see the comment in NativeProcessLinux::AllocateMemory and lldb_private::InferiorCallMmap / Process::RunThreadPlan for the full details of what it needs to do. I'm not completely familiar with how the interaction between lldb and lldb-server works, and I can try to implement a function call with a NativeProcessLinux (preserving the previous state) but it seems like this will be reimplementing a lot of functionality to do so naively. Any thoughts? Is there a simple way to call malloc in the context of the NativeProcessLinux and get the return that I'm missing?
>
> On Fri, May 1, 2015 at 2:27 PM, Greg Clayton <gclayton at apple.com> wrote:
>
> > On May 1, 2015, at 7:00 AM, Robert Flack <flackr at gmail.com> wrote:
> >
> > Thanks Greg for the suggestions, I'll start working on that and put up a patch when I have something working.
> >
> > Even doing it this way though, I imagine we'll still want to be able to load debug info for standard libraries if we can find it. I'm guessing we can only call functions from the symbol table if we have the function spec provided elsewhere (i.e. from a header file) or we know exactly what the function arguments are (as was the case in lldb_private::InferiorCallMmap).
>
> mmap is a symbol every app needs to be able to link to. We don't need debug info to call it. We just need to be able to find the symbol for it so we know where the code for mmap is.
>
> > On Fri, May 1, 2015 at 3:47 AM, Pavel Labath <labath at google.com> wrote:
> > Hi,
> >
> > this may not be so important if we choose the approach Greg suggests
> > (which sounds like a good idea), but in any case, I wanted to say that
> > I don't think that we should be depending on the debug symbols in libc
> > for a basic functionality like this. I think it's not safe to assume
> > that debug symbols will be available on every machine. And in any
> > case, mmap is a public symbol in libc, so it should be possible to
> > find it without debug symbols:
> > $ objdump -T /lib/libc-2.20.so | grep mmap
> > 00000000000e3670 w DF .text 0000000000000024 GLIBC_2.2.5 mmap64
> > 00000000000e3670 w DF .text 0000000000000024 GLIBC_2.2.5 mmap
> >
> > What does objdump produce on your machine? If mmap is there, and lldb
> > is not finding it then I think we should find out why...
> >
> > I'd like to not depend on the debug symbols, but we should definitely load them if we can find them (as we do when running locally).
>
> Agreed. We don't require them, but it will be nice to have them.
>
> > The symbol table is in the stripped .so (readelf -s or objdump -T show the symbol entries for mmap), but we're calling SymbolFileSymtab::FindFunctions which returns 0 because we don't have the full method info. When called from lldb_private::InferiorCallMmap, it seems like all we need is the base address which we could get that from the symbol table. To support general function calls into those functions though, as far as I can tell we can't tell from the symbol table what arguments are required, only the base address. I haven't tested this yet but I'm hoping that if the system header file for that function is included we'll find the function spec in the target's debug info.
>
> Again, if "mmap" can be looked up via:
>
> void *mmap_address = dlsym(..., "mmap");
>
> Then there must be a public symbol available in the libc.so. If LLDB doesn't see this, we need to fix that, even without debug symbols.
>
>
>
> >
> > cheers,
> > pl
> >
> >
> > On 30 April 2015 at 23:53, Greg Clayton <gclayton at apple.com> wrote:
> > >
> > >> On Apr 30, 2015, at 3:36 PM, Robert Flack <flackr at gmail.com> wrote:
> > >>
> > >> Hi,
> > >>
> > >> I've been looking into why expression evaluation is failing when targeting a remote Linux machine from Mac lldb host and it seems there are only 2 distinct problems remaining - both around memory allocation:
> > >>
> > >> 1. Can't find symbol for mmap.
> > >> 2. Once found, lldb is calling mmap with incorrect constant values for MAP_ANON.
> > >>
> > >> For problem 1, the library being linked against (e.g. /lib/x86_64-linux-gnu/libc-2.19.so) is copied into a local module cache, but we don't copy the unstripped library in /usr/lib/debug/lib/x86_64-linux-gnu/libc-2.19.so (I'm assuming we can't call mmap from the symtab file given SymbolFileSymtab::FindFunctions returns 0). To avoid having to duplicate the symbol discovery (in Symbols::LocateExecutableSymbolFile) we should probably ask lldb-platform on the target to find the symbol files for the current target (I'm thinking Platform::ResolveSymbolFile looks like the right place).
> > >>
> > >> For problem 2, we're building the argument list to mmap and the constant for MAP_ANON on macosx is 0x1000 whereas for linux it's 0x20. I'm not sure what the right way to fix this is, I could imagine asking Platform to allocate memory, but this would likely be an involved change, or perhaps being able to ask platform for various OS specific const values which would be hard-coded into it when built for the target.
> > >>
> > > So we need to implement the allocate and deallocate memory packets in lldb-server. It seems we have it implemented in the client, but not in the server:
> > >
> > > addr_t
> > > GDBRemoteCommunicationClient::AllocateMemory (size_t size, uint32_t permissions)
> > > {
> > > if (m_supports_alloc_dealloc_memory != eLazyBoolNo)
> > > {
> > > m_supports_alloc_dealloc_memory = eLazyBoolYes;
> > > char packet[64];
> > > const int packet_len = ::snprintf (packet, sizeof(packet), "_M%" PRIx64 ",%s%s%s",
> > > (uint64_t)size,
> > > permissions & lldb::ePermissionsReadable ? "r" : "",
> > > permissions & lldb::ePermissionsWritable ? "w" : "",
> > > permissions & lldb::ePermissionsExecutable ? "x" : "");
> > > assert (packet_len < (int)sizeof(packet));
> > > StringExtractorGDBRemote response;
> > > if (SendPacketAndWaitForResponse (packet, packet_len, response, false) == PacketResult::Success)
> > > {
> > > if (response.IsUnsupportedResponse())
> > > m_supports_alloc_dealloc_memory = eLazyBoolNo;
> > > else if (!response.IsErrorResponse())
> > > return response.GetHexMaxU64(false, LLDB_INVALID_ADDRESS);
> > > }
> > > else
> > > {
> > > m_supports_alloc_dealloc_memory = eLazyBoolNo;
> > > }
> > > }
> > > return LLDB_INVALID_ADDRESS;
> > > }
> > >
> > > bool
> > > GDBRemoteCommunicationClient::DeallocateMemory (addr_t addr)
> > > {
> > > if (m_supports_alloc_dealloc_memory != eLazyBoolNo)
> > > {
> > > m_supports_alloc_dealloc_memory = eLazyBoolYes;
> > > char packet[64];
> > > const int packet_len = ::snprintf(packet, sizeof(packet), "_m%" PRIx64, (uint64_t)addr);
> > > assert (packet_len < (int)sizeof(packet));
> > > StringExtractorGDBRemote response;
> > > if (SendPacketAndWaitForResponse (packet, packet_len, response, false) == PacketResult::Success)
> > > {
> > > if (response.IsUnsupportedResponse())
> > > m_supports_alloc_dealloc_memory = eLazyBoolNo;
> > > else if (response.IsOKResponse())
> > > return true;
> > > }
> > > else
> > > {
> > > m_supports_alloc_dealloc_memory = eLazyBoolNo;
> > > }
> > > }
> > > return false;
> > > }
> > >
> > > Then you call mmap yourself on the native machine in lldb-server instead of trying to know what enums will work.
> > >
> > > We actually need to ask the PlatformLinux to run an allocate/deallocate memory and hand it a process. So we can add the following to lldb_private::Platform:
> > >
> > > virtual bool
> > > SupportsMemoryAllocation();
> > >
> > > virtual lldb::addr_t
> > > AllocateMemory (lldb_private::Process *process, size_t size, uint32_t permissions, Error &error);
> > >
> > >
> > > virtual Error
> > > DeallocateMemory (lldb_private::Process *process, lldb::addr_t ptr);
> > >
> > > Then the lldb_private::Process can get the current platform and ask it if it supports allocating memory, and if so call the Platform::AllocateMemory()/Platform:: DeallocateMemory().
> > >
> > > Then the PlatformLinux can "do the right thing" and use the right defines.
> > >
> > >> Anyways, I wanted to send this out to see if anyone had any thoughts on either of these issues or was already working on them. I have verified (by hacking in the correct const values for linux and placing debug libs in a path where they will be found) that this fixes expression evaluation (and 14 tests start passing) for mac->linux debugging.
> > >>
> > >> Thanks in advance for any suggestions,
> > >> Rob
> > >
> > > So my suggestion is to implement the memory allocation/deallocation in lldb-server since it runs natively and will avoid the problems we run into by trying to evaluate functions by calling them remotely using #define values from the current system...
> > >
> > >
> > >>
> > >> P.S. the 14 tests passing mac->linux by fixing this (for other people looking at cross platform tests):
> > >> Test11588.py
> > >> TestAnonymous.py
> > >> TestBreakpointConditions.py
> > >> TestCPPStaticMethods.py
> > >> TestCStrings.py
> > >> TestCallStdStringFunction.py
> > >> TestDataFormatterCpp.py
> > >> TestDataFormatterStdList.py
> > >> TestExprDoesntBlock.py
> > >> TestExprHelpExamples.py
> > >> TestFunctionTypes.py
> > >> TestPrintfAfterUp.py
> > >> TestSBValuePersist.py
> > >> TestSetValues.py
> > >>
> > >> _______________________________________________
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> > >> lldb-dev at cs.uiuc.edu
> > >> http://lists.cs.uiuc.edu/mailman/listinfo/lldb-dev
> > >
> > >
> > > _______________________________________________
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> >
>
>
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