On Wed, Jun 15, 2022 at 10:56 PM Andrii Nakryiko <andrii@xxxxxxxxxx> wrote: > > Perform the same virtual address to file offset translation that libbpf > is doing for executable ELF binaries also for shared libraries. > Currently libbpf is making a simplifying and sometimes wrong assumption > that for shared libraries relative virtual addresses inside ELF are > always equal to file offsets. > > Unfortunately, this is not always the case with LLVM's lld linker, which > now by default generates quite more complicated ELF segments layout. > E.g., for liburandom_read.so from selftests/bpf, here's an excerpt from > readelf output listing ELF segments (a.k.a. program headers): > > Type Offset VirtAddr PhysAddr FileSiz MemSiz Flg Align > PHDR 0x000040 0x0000000000000040 0x0000000000000040 0x0001f8 0x0001f8 R 0x8 > LOAD 0x000000 0x0000000000000000 0x0000000000000000 0x0005e4 0x0005e4 R 0x1000 > LOAD 0x0005f0 0x00000000000015f0 0x00000000000015f0 0x000160 0x000160 R E 0x1000 > LOAD 0x000750 0x0000000000002750 0x0000000000002750 0x000210 0x000210 RW 0x1000 > LOAD 0x000960 0x0000000000003960 0x0000000000003960 0x000028 0x000029 RW 0x1000 > > Compare that to what is generated by GNU ld (or LLVM lld's with extra > -znoseparate-code argument which disables this cleverness in the name of > file size reduction): > > Type Offset VirtAddr PhysAddr FileSiz MemSiz Flg Align > LOAD 0x000000 0x0000000000000000 0x0000000000000000 0x000550 0x000550 R 0x1000 > LOAD 0x001000 0x0000000000001000 0x0000000000001000 0x000131 0x000131 R E 0x1000 > LOAD 0x002000 0x0000000000002000 0x0000000000002000 0x0000ac 0x0000ac R 0x1000 > LOAD 0x002dc0 0x0000000000003dc0 0x0000000000003dc0 0x000262 0x000268 RW 0x1000 > > You can see from the first example above that for executable (Flg == "R E") > PT_LOAD segment (LOAD #2), Offset doesn't match VirtAddr columns. > And it does in the second case (GNU ld output). > > This is important because all the addresses, including USDT specs, > operate in a virtual address space, while kernel is expecting file > offsets when performing uprobe attach. So such mismatches have to be > properly taken care of and compensated by libbpf, which is what this > patch is fixing. > > Also patch clarifies few function and variable names, as well as updates > comments to reflect this important distinction (virtaddr vs file offset) > and to ephasize that shared libraries are not all that different from > executables in this regard. > > This patch also changes selftests/bpf Makefile to force urand_read and > liburand_read.so to be built with Clang and LLVM's lld (and explicitly > request this ELF file size optimization through -znoseparate-code linker > parameter) to validate libbpf logic and ensure regressions don't happen > in the future. I've bundled these selftests changes together with libbpf > changes to keep the above description tied with both libbpf and > selftests changes. > > Signed-off-by: Andrii Nakryiko <andrii@xxxxxxxxxx> > --- > tools/lib/bpf/usdt.c | 123 ++++++++++++++------------- What should be the Fixes tag here? Back to the beginning of this usdt.c file?
