Hi Richard, elfutils hackers, Fedora builds everything with -D_FORTIFY_SOURCE=2 which catches an issue with the bpf_disasm.c printf format strings. It can most easily be seen by compiling with CFLAGS="-g -Og -D_FORTIFY_SOURCE=2" and running the objdump testcase from run-disasm-bpf.sh. It will crash with: *** invalid %N$ use detected ***
This is caused by the following restriction on using the %N$ argument selection: There may be no gaps in the numbers of arguments specified using '$'; for example, if arguments 1 and 3 are specified, argument 2 must also be specified somewhere in the format string. (from man 3 printf) Of course that is precisely the point of using the %N$ arguments in the bpf format strings. Only use those arguments relevant for the instruction. I was hoping there was some trick to include the other arguments anyway, but give them a zero width or something, so they aren't actually included in the output. But there doesn't seem to be a way. Alternatively rearranging the arguments so optionally unused arguments always come last also doesn't seem easily possible since different instructions use different combinations of the dst_reg, src_reg, imm, off or jmp label. Maybe splitting the formats and arguments per instruction class might help? Does anybody see a trick to get around this format restriction? Cheers, Mark
