Re: [PATCH v2] bpf: avoid creating wrong symbols while parsing

[David Faust <david.faust@oracle.com>]

On 11/17/23 11:29, Jose E. Marchesi wrote:
> 
> Hi David.
> 
>> [ WAS: gas,bpf: cleanup bad symbols created while parsing
>>   v1: https://sourceware.org/pipermail/binutils/2023-November/130556.html
>>
>>   Changes from v1:
>>   - Rewrite patch to avoid inserting wrong symbols in the first place,
>>     rather than insert, remove, re-insert dance.
>>     Suggested by Jan Beulich. ]
>>
>> To support the "pseudo-C" asm dialect in BPF, the BPF parser must often
>> attempt multiple different templates for a single instruction. In some
>> cases this can cause the parser to incorrectly parse part of the
>> instruction opcode as an expression, which leads to the creation of a
>> new undefined symbol.
>>
>> Once the parser recognizes the error, the expression is discarded and it
>> tries again with a new instruction template. However, symbols created
>> during the process are added to the symbol table and are not removed
>> even if the expression is discarded.
>>
>> This is a problem for BPF: generally the assembled object will be loaded
>> directly to the Linux kernel, without being linked. These erroneous
>> parser-created symbols are rejected by the kernel BPF loader, and the
>> entire object is refused.
>>
>> This patch remedies the issue by tentatively creating symbols while
>> parsing instruction operands, and storing them in a temporary list
>> rather than immediately inserting them into the symbol table. Later,
>> after the parser is sure that it has correctly parsed the instruction,
>> those symbols are committed to the real symbol table.
>>
>> This approach is modeled directly after Jan Beulich's patch for RISC-V:
>>
>>   commit 7a29ee290307087e1749ce610207e93a15d0b78d
>>   RISC-V: adjust logic to avoid register name symbols
>>
>> Many thanks to Jan for recognizing the problem as similar, and pointing
>> me to that patch.
>>
>> Tested on x86_64-linux-gnu host for bpf-unknown-none target.
>>
>> gas/
>>
>> 	* config/tc-bpf.c (parsing_insn_operands): New.
>> 	(parse_expression): Set it here.
>> 	(deferred_sym_rootP, deferred_sym_lastP): New.
>> 	(orphan_sym_rootP, orphan_sym_lastP): New.
>> 	(bpf_parse_name): New.
>> 	(parse_error): Clear deferred symbol list on error.
>> 	(md_assemble): Clear parsing_insn_operands. Commit deferred
>> 	symbols to symbol table on successful parse.
>> 	* config/tc-bpf.h (md_parse_name): Define to...
>> 	(bpf_parse_name): ...this. New prototype.
>> 	* testsuite/gas/bpf/asm-extra-sym-1.s: New test source.
>> 	* testsuite/gas/bpf/asm-extra-sym-1.d: New test.
>> 	* testsuite/gas/bpf/bpf.exp: Run new test.
>> ---
>>  gas/config/tc-bpf.c                     | 92 +++++++++++++++++++++++++
>>  gas/config/tc-bpf.h                     |  4 ++
>>  gas/testsuite/gas/bpf/asm-extra-sym-1.d |  7 ++
>>  gas/testsuite/gas/bpf/asm-extra-sym-1.s |  1 +
>>  gas/testsuite/gas/bpf/bpf.exp           |  3 +
>>  5 files changed, 107 insertions(+)
>>  create mode 100644 gas/testsuite/gas/bpf/asm-extra-sym-1.d
>>  create mode 100644 gas/testsuite/gas/bpf/asm-extra-sym-1.s
>>
>> diff --git a/gas/config/tc-bpf.c b/gas/config/tc-bpf.c
>> index fd4144a354b..3122f80804a 100644
>> --- a/gas/config/tc-bpf.c
>> +++ b/gas/config/tc-bpf.c
>> @@ -1223,6 +1223,7 @@ add_relaxed_insn (struct bpf_insn *insn, expressionS *exp)
>>     See md_operand below to see how exp_parse_failed is used.  */
>>  
>>  static int exp_parse_failed = 0;
>> +static bool parsing_insn_operands = false;
>>  
>>  static char *
>>  parse_expression (char *s, expressionS *exp)
>> @@ -1230,6 +1231,9 @@ parse_expression (char *s, expressionS *exp)
>>    char *saved_input_line_pointer = input_line_pointer;
>>    char *saved_s = s;
>>  
>> +  /* Wake up bpf_parse_name before the call to expression ().  */
>> +  parsing_insn_operands = true;
>> +
>>    exp_parse_failed = 0;
>>    input_line_pointer = s;
>>    expression (exp);
>> @@ -1251,6 +1255,71 @@ parse_expression (char *s, expressionS *exp)
>>    return s;
>>  }
>>  
>> +/* Symbols created by this parse, but not yet committed to the real
>> +   symbol table.  */
>> +static symbolS *deferred_sym_rootP;
>> +static symbolS *deferred_sym_lastP;
>> +
>> +/* Symbols discarded by a previous parse.  Symbols cannot easily be freed
>> +   after creation, so try to recycle.  */
>> +static symbolS *orphan_sym_rootP;
>> +static symbolS *orphan_sym_lastP;
>> +
>> +/* Implement md_parse_name hook.  Handles any symbol found in an expression.
>> +   This allows us to tentatively create symbols, before we know for sure
>> +   whether the parser is using the correct template for an instruction.
>> +   If we end up keeping the instruction, the deferred symbols are committed
>> +   to the real symbol table. This approach is modeled after the riscv port.  */
>> +
>> +bool
>> +bpf_parse_name (const char *name, expressionS *exp, enum expr_mode mode)
>> +{
>> +  symbolS *sym;
>> +
>> +  /* If we aren't currently parsing an instruction, don't do anything.
>> +     This prevents tampering with operands to directives.  */
>> +  if (!parsing_insn_operands)
>> +    return false;
>> +
>> +  gas_assert (mode == expr_normal);
>> +
>> +  if (symbol_find (name) != NULL)
>> +    return false;
> 
> Is this check correct?
> 
> I see that expr.c:operand calls symbol_find_or_make right after calling
> md_parse_name (if the later is defined) and actually installs the
> resulting symbol in expressionP even if it was already defined.  That is
> not done if md_parse_name returns `false'.

Hmm, I think you may have the logic in operand () flipped.
If md_parse_name returns 'false' then symbol_find_or_make is called;
if it returns 'true' then the 'break' is hit:

#ifdef md_parse_name
	...
	  if (md_parse_name (name, expressionP, mode, &c))
	    {
	      restore_line_pointer (c);
	      break;
	    }
#endif
	  symbolP = symbol_find_or_make (name);
	...

That is what we want to happen - if the symbol_find in parse_name
returns non-NULL, then we know a real symbol already exists for the
name we are looking at, and we should use it. In that case,
md_parse_name returns false, so operand () calls symbol_find_or_make,
which finds the extant symbol and installs it in the expression.

> 
>> +
>> +  for (sym = deferred_sym_rootP; sym; sym = symbol_next (sym))
>> +    if (strcmp (name, S_GET_NAME (sym)) == 0)
>> +      break;
>> +
>> +  /* Tentatively create a symbol.  */
>> +  if (!sym)
>> +    {
>> +      /* See if we can reuse a symbol discarded by a previous parse.
>> +	 This may be quite common, for example when trying multiple templates
>> +	 for an instruction with the first reference to a valid symbol.  */
>> +      for (sym = orphan_sym_rootP; sym; sym = symbol_next (sym))
>> +	if (strcmp (name, S_GET_NAME (sym)) == 0)
>> +	  {
>> +	    symbol_remove (sym, &orphan_sym_rootP, &orphan_sym_lastP);
>> +	    break;
>> +	  }
>> +
>> +      if (!sym)
>> +	  sym = symbol_create (name, undefined_section, &zero_address_frag, 0);
>> +
>> +      /* Add symbol to the deferred list.  If we commit to the isntruction,
>> +	 then the symbol will be inserted into to the real symbol table at
>> +	 that point (in md_assemble).  */
>> +      symbol_append (sym, deferred_sym_lastP, &deferred_sym_rootP,
>> +		     &deferred_sym_lastP);
>> +    }
>> +
>> +  exp->X_op = O_symbol;
>> +  exp->X_add_symbol = sym;
>> +  exp->X_add_number = 0;
>> +
>> +  return true;
>> +}
>> +
>>  /* Parse a BPF register name and return the corresponding register
>>     number.  Return NULL in case of parse error, or a pointer to the
>>     first character in S that is not part of the register name.  */
>> @@ -1317,6 +1386,16 @@ parse_error (int length, const char *fmt, ...)
>>        va_end (args);
>>        partial_match_length = length;
>>      }
>> +
>> +  /* Discard deferred symbols from the failed parse.  They may potentially
>> +     be reused in the future from the orphan list.  */
>> +  while (deferred_sym_rootP)
>> +    {
>> +      symbolS *sym = deferred_sym_rootP;
>> +      symbol_remove (sym, &deferred_sym_rootP, &deferred_sym_lastP);
>> +      symbol_append (sym, orphan_sym_lastP, &orphan_sym_rootP,
>> +		     &orphan_sym_lastP);
>> +    }
>>  }
>>  
>>  /* Assemble a machine instruction in STR and emit the frags/bytes it
>> @@ -1606,6 +1685,10 @@ md_assemble (char *str ATTRIBUTE_UNUSED)
>>          }
>>      }
>>  
>> +  /* Mark that we are no longer parsing an instruction, bpf_parse_name does
>> +     not interfere with symbols in e.g. assembler directives.  */
>> +  parsing_insn_operands = false;
>> +
>>    if (opcode == NULL)
>>      {
>>        as_bad (_("unrecognized instruction `%s'"), str);
>> @@ -1622,6 +1705,15 @@ md_assemble (char *str ATTRIBUTE_UNUSED)
>>  
>>  #undef PARSE_ERROR
>>  
>> +  /* Commit any symbols created while parsing the instruction.  */
>> +  while (deferred_sym_rootP)
>> +    {
>> +      symbolS *sym = deferred_sym_rootP;
>> +      symbol_remove (sym, &deferred_sym_rootP, &deferred_sym_lastP);
>> +      symbol_append (sym, symbol_lastP, &symbol_rootP, &symbol_lastP);
>> +      symbol_table_insert (sym);
>> +    }
>> +
>>    /* Generate the frags and fixups for the parsed instruction.  */
>>    if (do_relax && isa_spec >= BPF_V4 && insn.is_relaxable)
>>      {
>> diff --git a/gas/config/tc-bpf.h b/gas/config/tc-bpf.h
>> index 9fb71eddd14..06096ef5926 100644
>> --- a/gas/config/tc-bpf.h
>> +++ b/gas/config/tc-bpf.h
>> @@ -51,6 +51,10 @@
>>     a jump to offset 0 means jump to the next instruction.  */
>>  #define md_single_noop_insn "ja 0"
>>  
>> +#define md_parse_name(name, exp, mode, c) \
>> +  bpf_parse_name (name, exp, mode)
>> +bool bpf_parse_name (const char *, struct expressionS *, enum expr_mode);
>> +
>>  #define TC_EQUAL_IN_INSN(c, s) bpf_tc_equal_in_insn ((c), (s))
>>  extern bool bpf_tc_equal_in_insn (int, char *);
>>  
>> diff --git a/gas/testsuite/gas/bpf/asm-extra-sym-1.d b/gas/testsuite/gas/bpf/asm-extra-sym-1.d
>> new file mode 100644
>> index 00000000000..113750dd3fd
>> --- /dev/null
>> +++ b/gas/testsuite/gas/bpf/asm-extra-sym-1.d
>> @@ -0,0 +1,7 @@
>> +#as: -EL -mdialect=pseudoc
>> +#nm: --numeric-sort
>> +#source: asm-extra-sym-1.s
>> +#name: BPF pseudoc no extra symbols 1
>> +
>> +# Note: there should be no output from nm.
>> +# Previously a bug in the BPF parser created an UND '*' symbol.
>> diff --git a/gas/testsuite/gas/bpf/asm-extra-sym-1.s b/gas/testsuite/gas/bpf/asm-extra-sym-1.s
>> new file mode 100644
>> index 00000000000..2cfa605a259
>> --- /dev/null
>> +++ b/gas/testsuite/gas/bpf/asm-extra-sym-1.s
>> @@ -0,0 +1 @@
>> +    r2 = *(u32*)(r1 + 8)
>> diff --git a/gas/testsuite/gas/bpf/bpf.exp b/gas/testsuite/gas/bpf/bpf.exp
>> index 80f5a1dbc2d..fcbeccd8ecd 100644
>> --- a/gas/testsuite/gas/bpf/bpf.exp
>> +++ b/gas/testsuite/gas/bpf/bpf.exp
>> @@ -72,4 +72,7 @@ if {[istarget bpf*-*-*]} {
>>      run_dump_test disp16-overflow-relax
>>      run_dump_test disp32-overflow
>>      run_dump_test imm32-overflow
>> +
>> +    # Test that parser does not create undefined symbols
>> +    run_dump_test asm-extra-sym-1
>>  }