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bpf_disasm format strings vs FORTIFY_SOURCE
- From: Mark Wielaard <mjw at redhat dot com>
- To: elfutils-devel at lists dot fedorahosted dot org
- Date: Mon, 08 Aug 2016 01:09:45 +0200
- Subject: bpf_disasm format strings vs FORTIFY_SOURCE
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
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
Does anybody see a trick to get around this format restriction?