From mboxrd@z Thu Jan 1 00:00:00 1970 X-Msuck: nntp://news.gmane.org/gmane.linux.lib.musl.general/7636 Path: news.gmane.org!not-for-mail From: Alex Dowad Newsgroups: gmane.linux.lib.musl.general Subject: [PATCH v3] Build process uses script to add CFI directives to x86 asm Date: Wed, 13 May 2015 19:54:39 +0200 Message-ID: <1431539679-4265-1-git-send-email-alexinbeijing@gmail.com> Reply-To: musl@lists.openwall.com NNTP-Posting-Host: plane.gmane.org X-Trace: ger.gmane.org 1431539730 16742 80.91.229.3 (13 May 2015 17:55:30 GMT) X-Complaints-To: usenet@ger.gmane.org NNTP-Posting-Date: Wed, 13 May 2015 17:55:30 +0000 (UTC) To: musl@lists.openwall.com Original-X-From: musl-return-7648-gllmg-musl=m.gmane.org@lists.openwall.com Wed May 13 19:55:30 2015 Return-path: Envelope-to: gllmg-musl@m.gmane.org Original-Received: from mother.openwall.net ([195.42.179.200]) by plane.gmane.org with smtp (Exim 4.69) (envelope-from ) id 1YsasS-0006VZ-HN for gllmg-musl@m.gmane.org; Wed, 13 May 2015 19:55:28 +0200 Original-Received: (qmail 11295 invoked by uid 550); 13 May 2015 17:55:26 -0000 Mailing-List: contact musl-help@lists.openwall.com; run by ezmlm Precedence: bulk List-Post: List-Help: List-Unsubscribe: List-Subscribe: Original-Received: (qmail 10061 invoked from network); 13 May 2015 17:55:00 -0000 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=gmail.com; s=20120113; h=from:to:subject:date:message-id; bh=w0bShrMESyHp8JfJOOdva9Wm6aWglVlG1Xj4PE+kzXM=; b=yF1JIxN3NuXXWp5x6kKireTAaufuBZhCVYWAnw2Ogth9K5oqFqtqTG2zxOgAyJi15B OTr8/sYwuRprpJdvL97K54PgtznxMS72Er9nJy1VFEYrmoaJdwtgych0H/j5T3KXwUEN /0sM3ZTv6WZ/L61A0Mg83wBx7eAW7C4wqHgpQzqcNI6ZFy0vgO2VEDQJAMnA0zeSg7zr a3xZ/np3K1qCk5G7HS1FOnZtOpwcRS810sfF9t+073baWLGbeR48J6gtiRJf0YM39MlU Wkl2GtenIjXzHwPVmtnadWJHupxAwFu7Fssuqk99eqMx+g4ahro9t49q6ZCSKIOV6P/k JlJg== X-Received: by 10.194.223.66 with SMTP id qs2mr289573wjc.6.1431539689308; Wed, 13 May 2015 10:54:49 -0700 (PDT) X-Mailer: git-send-email 2.0.0.GIT Xref: news.gmane.org gmane.linux.lib.musl.general:7636 Archived-At: Some functions implemented in asm need to use EBP for purposes other than acting as a frame pointer. (Notably, it is used for the 6th argument to syscalls with 6 arguments.) Without frame pointers, GDB can only show backtraces if it gets CFI information from a .debug_frame or .eh_frame ELF section. Rather than littering our asm with ugly .cfi directives, use an awk script to insert them in the right places during the build process, so GDB can keep track of where the current stack frame is relative to the stack pointer. This means GDB can produce beautiful stack traces at any given point when single-stepping through asm functions. Additionally, when registers are saved on the stack and later overwritten, emit .cfi directives so GDB will know where they were saved relative to the stack pointer. This way, when you look back up the stack from within an asm function, you can still reliably print the values of local variables in the caller. If this awk script were to understand every possible wild and crazy contortion that an asm programmer can do with the stack and registers, and always emit the exact .cfi directives needed for GDB to know what the register values were in the preceding stack frame, it would necessarily be as complex as a full x86 emulator. That way lies madness. Hence, we assume that the stack pointer will _only_ ever be adjusted using push/pop or else add/sub with a constant. We do not attempt to detect every possible way that a register value could be saved for later use. --- Dear muslers, The AWK script here has been tweaked up and made more robust in response to suggestions from Szabolcs Nagy. I've noticed that using tempfiles for the augmented asm has a drawback: In the source file/line debugging info generated by the assembler, it records the source file as "/tmp/". Then, when you try to debug a program which is linked against the resulting musl, GDB tries to open "/tmp/" to show in the source window. Suggestions?? Perhaps generate .cfi.s files as Szabolcs suggested?? Thanks, Alex Dowad Makefile | 2 +- tools/add-cfi.awk.i386 | 176 +++++++++++++++++++++++++++++++++++++++++++++++++ tools/aswrap.sh | 15 +++++ 3 files changed, 192 insertions(+), 1 deletion(-) create mode 100644 tools/add-cfi.awk.i386 create mode 100755 tools/aswrap.sh diff --git a/Makefile b/Makefile index 6559295..9aefd62 100644 --- a/Makefile +++ b/Makefile @@ -118,7 +118,7 @@ $(foreach s,$(wildcard src/*/$(ARCH)*/*.s),$(eval $(call mkasmdep,$(s)))) $(CC) $(CFLAGS_ALL_STATIC) -c -o $@ $(dir $<)$(shell cat $<) %.o: $(ARCH)/%.s - $(CC) $(CFLAGS_ALL_STATIC) -c -o $@ $< + tools/aswrap.sh $< $@ $(ARCH) "$(CC) $(CFLAGS_ALL_STATIC)" %.o: %.c $(GENH) $(IMPH) $(CC) $(CFLAGS_ALL_STATIC) -c -o $@ $< diff --git a/tools/add-cfi.awk.i386 b/tools/add-cfi.awk.i386 new file mode 100644 index 0000000..116fba8 --- /dev/null +++ b/tools/add-cfi.awk.i386 @@ -0,0 +1,176 @@ +# Insert GAS CFI directives ("control frame information") into x86-32 asm input +# +# CFI directives tell the assembler how to generate "stack frame" debug info +# This information can tell a debugger (like gdb) how to find the current stack +# frame at any point in the program code, and how to find the values which +# various registers had at higher points in the call stack +# With this information, the debugger can show a backtrace, and you can move up +# and down the call stack and examine the values of local variables + +BEGIN { + # don't put CFI data in the .eh_frame ELF section (which we don't keep) + print ".cfi_sections .debug_frame" + + # only emit CFI directives inside a function + in_function = 0 +} + +function hex2int(str, i) { + str = tolower(str) + + for (i = 1; i <= 16; i++) { + char = substr("0123456789abcdef", i, 1) + lookup[char] = i-1 + } + + result = 0 + for (i = 1; i <= length(str); i++) { + result = result * 16 + char = substr(str, i, 1) + result = result + lookup[char] + } + return result +} + +function get_const1() { + # for instructions with 2 operands, get 1st operand (assuming it is constant) + match($0, /-?(0x[0-9a-fA-F]+|[0-9]+),/) + return parse_const(substr($0, RSTART, RLENGTH-1)) +} +function parse_const(const) { + if (substr(const, 1, 1) == "-") { + if (substr(const, 2, 2) == "0x") { + return -hex2int(substr(const, 4, length(const)-3)) + } else { + return const + } + } else { + if (substr(const, 1, 2) == "0x") { + return hex2int(substr(const, 3, length(const)-2)) + } else { + return const + } + } +} +function get_reg() { + # only use if you already know there is 1 and only 1 register + match($0, /%e(ax|bx|cx|dx|si|di|bp)/) + return substr($0, RSTART+1, 3) +} +function get_reg1() { + # for instructions with 2 operands, get 1st operand (assuming it is register) + match($0, /%e(ax|bx|cx|dx|si|di|bp)\s*,/) + return substr($0, RSTART+1, 3) +} +function get_reg2() { + # for instructions with 2 operands, get 2nd operand (assuming it is register) + match($0, /,\s*%e(ax|bx|cx|dx|si|di|bp)/) + return substr($0, RSTART+RLENGTH-3, 3) +} + +function adjust_sp_offset(delta) { + if (in_function) { + printf ".cfi_adjust_cfa_offset %d\n", delta + } +} + +{ print } + +/^.global\s+\w+/ { + globals[$2] = 1 +} +/^\w+:/ { + label = substr($1, 1, length($1)-1) # drop trailing : + + if (globals[label]) { + if (in_function) + print ".cfi_endproc" + + in_function = 1 + print ".cfi_startproc" + + for (register in saved) + delete saved[register] + for (register in dirty) + delete dirty[register] + } +} + +# KEEPING UP WITH THE STACK POINTER +# We do NOT attempt to understand foolish and ridiculous tricks like stashing +# the stack pointer and then using %esp as a scratch register, or bitshifting +# it or taking its square root or anything stupid like that. +# %esp should only be adjusted by pushing/popping or adding/subtracting constants +# +/pushl?/ { + if (match($0, /\s+%(ax|bx|cx|dx|di|si|bp|sp)/)) + adjust_sp_offset(2) + else + adjust_sp_offset(4) +} +/popl?/ { + if (match($0, /\s+%(ax|bx|cx|dx|di|si|bp|sp)/)) + adjust_sp_offset(-2) + else + adjust_sp_offset(-4) +} +/addl?\s+\$-?(0x[0-9a-fA-F]+|[0-9]+),\s*%esp/ { adjust_sp_offset(-get_const1()) } +/subl?\s+\$-?(0x[0-9a-fA-F]+|[0-9]+),\s*%esp/ { adjust_sp_offset(get_const1()) } + +# TRACKING REGISTER VALUES FROM THE PREVIOUS STACK FRAME +# +/pushl?\s+%e(ax|bx|cx|dx|si|di|bp)/ { # don't match "push (%reg)" + # if a register is being pushed, and its value has not changed since the + # beginning of this function, the pushed value can be used when printing + # local variables at the next level up the stack + # emit '.cfi_rel_offset' for that + + if (in_function) { + register = get_reg() + if (!saved[register] && !dirty[register]) { + printf ".cfi_rel_offset %s,0\n", register + saved[register] = 1 + } + } +} + +/movl?\s+%e(ax|bx|cx|dx|si|di|bp),\s*-?(0x[0-9a-fA-F]+|[0-9]+)?\(%esp\)/ { + if (in_function) { + register = get_reg() + if (match($0, /-?(0x[0-9a-fA-F]+|[0-9]+)\(%esp\)/)) { + offset = parse_const(substr($0, RSTART, RLENGTH-6)) + } else { + offset = 0 + } + if (!saved[register] && !dirty[register]) { + printf ".cfi_rel_offset %s,%d\n", register, offset + saved[register] = 1 + } + } +} + +# IF REGISTER VALUES ARE UNCEREMONIOUSLY TRASHED +# ...then we want to know about it. +# +function trashed(register) { + if (in_function && !saved[register] && !dirty[register]) { + printf ".cfi_undefined %s\n", register + } + dirty[register] = 1 +} +# this does NOT exhaustively check for all possible instructions which could +# overwrite a register value inherited from the caller (just the common ones) +/mov.*,%e(ax|bx|cx|dx|si|di|bp)/ { trashed(get_reg2()) } +/(add|addl|sub|subl|and|or|xor|lea|sal|sar|shl|shr)\s+%e(ax|bx|cx|dx|si|di|bp),/ { + trashed(get_reg1()) +} +/i?mul\s+[^,]*$/ { trashed("eax"); trashed("edx") } +/i?mul\s+%e(ax|bx|cx|dx|si|di|bp),/ { trashed(get_reg1()) } +/^(\w+:)?\s*i?div/ { trashed("eax"); trashed("edx") } +/(dec|inc|not|neg|pop)\s+%e(ax|bx|cx|dx|si|di|bp)/ { trashed(get_reg()) } +/^(\w+:)\s*cpuid/ { trashed("eax"); trashed("ebx"); trashed("ecx"); trashed("edx") } + +END { + if (in_function) + print ".cfi_endproc" +} \ No newline at end of file diff --git a/tools/aswrap.sh b/tools/aswrap.sh new file mode 100755 index 0000000..0afbd4e --- /dev/null +++ b/tools/aswrap.sh @@ -0,0 +1,15 @@ +#!/bin/sh +# Run assembler to produce an object file, optionally applying other pre-processing steps +input=$1 +output=$2 +arch=$3 +as=$4 + +if [ -f "tools/add-cfi.awk.$arch" ]; then + tmpfile=$(mktemp -t musl-aswrap-XXXXXX) + awk -f tools/add-cfi.awk.$arch $input >$tmpfile + mv $tmpfile $tmpfile.s + input=$tmpfile.s +fi + +$as -c -o $output $input \ No newline at end of file -- 2.0.0.GIT