From: Simon Marchi Date: Fri, 4 Dec 2020 21:44:55 +0000 (-0500) Subject: gdb: use two displaced step buffers on amd64/Linux X-Git-Url: https://git.libre-soc.org/?a=commitdiff_plain;h=372ff58fdaf3df0616db77e1619da1ad3bd4b0e0;p=binutils-gdb.git gdb: use two displaced step buffers on amd64/Linux As observed on a binary compiled on AMD64 Ubuntu 20.04, against glibc 2.31 (I think it's the libc that provides this startup code, right?), there are enough bytes at the executable's entry point to hold more than one displaced step buffer. gdbarch_max_insn_length is 16, and the code at _start looks like: 0000000000001040 <_start>: 1040: f3 0f 1e fa endbr64 1044: 31 ed xor %ebp,%ebp 1046: 49 89 d1 mov %rdx,%r9 1049: 5e pop %rsi 104a: 48 89 e2 mov %rsp,%rdx 104d: 48 83 e4 f0 and $0xfffffffffffffff0,%rsp 1051: 50 push %rax 1052: 54 push %rsp 1053: 4c 8d 05 56 01 00 00 lea 0x156(%rip),%r8 # 11b0 <__libc_csu_fini> 105a: 48 8d 0d df 00 00 00 lea 0xdf(%rip),%rcx # 1140 <__libc_csu_init> 1061: 48 8d 3d c1 00 00 00 lea 0xc1(%rip),%rdi # 1129
1068: ff 15 72 2f 00 00 callq *0x2f72(%rip) # 3fe0 <__libc_start_main@GLIBC_2.2.5> 106e: f4 hlt 106f: 90 nop The two buffers would occupy [0x1040, 0x1060). I checked on Alpine, which uses the musl C library, the startup code looks like: 0000000000001048 <_start>: 1048: 48 31 ed xor %rbp,%rbp 104b: 48 89 e7 mov %rsp,%rdi 104e: 48 8d 35 e3 2d 00 00 lea 0x2de3(%rip),%rsi # 3e38 <_DYNAMIC> 1055: 48 83 e4 f0 and $0xfffffffffffffff0,%rsp 1059: e8 00 00 00 00 callq 105e <_start_c> 000000000000105e <_start_c>: 105e: 48 8b 37 mov (%rdi),%rsi 1061: 48 8d 57 08 lea 0x8(%rdi),%rdx 1065: 45 31 c9 xor %r9d,%r9d 1068: 4c 8d 05 47 01 00 00 lea 0x147(%rip),%r8 # 11b6 <_fini> 106f: 48 8d 0d 8a ff ff ff lea -0x76(%rip),%rcx # 1000 <_init> 1076: 48 8d 3d 0c 01 00 00 lea 0x10c(%rip),%rdi # 1189
107d: e9 9e ff ff ff jmpq 1020 <__libc_start_main@plt> Even though there's a _start_c symbol, it all appears to be code that runs once at the very beginning of the program, so it looks fine if the two buffers occupy [0x1048, 0x1068). One important thing I discovered while doing this is that when debugging a dynamically-linked executable, breakpoints in the shared library loader are hit before executing the _start code, and these breakpoints may be displaced-stepped. So it's very important that the buffer bytes are restored properly after doing the displaced steps, otherwise the _start code will be corrupted once we try to execute it. Another thing that made me think about is that library constructors (as in `__attribute__((constructor))`) run before _start. And they are free to spawn threads. What if one of these threads executes a displaced step, therefore changing the bytes at _start, while the main thread executes _start? That doesn't sound good and I don't know how we could prevent it. But this is a problem that predates the current patch. Even when stress-testing the implementation, by making many threads do displaced steps over and over, I didn't see a significant performance (I confirmed that the two buffers were used by checking the "set debug displaced" logs though). However, this patch mostly helps make the feature testable by anybody with an AMD64/Linux machine, so I think it's useful. gdb/ChangeLog: * amd64-linux-tdep.c (amd64_linux_init_abi): Pass 2 as the number of displaced step buffers. Change-Id: Ia0c96ea0fcda893f4726df6fdac7be5214620112 --- diff --git a/gdb/ChangeLog b/gdb/ChangeLog index 514bf6d0c53..8252c71452b 100644 --- a/gdb/ChangeLog +++ b/gdb/ChangeLog @@ -1,3 +1,8 @@ +2020-12-04 Simon Marchi + + * amd64-linux-tdep.c (amd64_linux_init_abi): Pass 2 as the + number of displaced step buffers. + 2020-12-04 Simon Marchi * displaced-stepping.h (struct displaced_step_buffer): Rename diff --git a/gdb/amd64-linux-tdep.c b/gdb/amd64-linux-tdep.c index 60707ed7aaf..4f2d83f0286 100644 --- a/gdb/amd64-linux-tdep.c +++ b/gdb/amd64-linux-tdep.c @@ -1880,7 +1880,7 @@ amd64_linux_init_abi (struct gdbarch_info info, struct gdbarch *gdbarch) if (!valid_p) return; - amd64_linux_init_abi_common (info, gdbarch, 1); + amd64_linux_init_abi_common (info, gdbarch, 2); /* Initialize the amd64_linux_record_tdep. */ /* These values are the size of the type that will be used in a system