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Date: Wed, 08 Aug 2012 15:57:15 +0200
From: musl <b.brezillon.musl@...il.com>
To: musl@...ts.openwall.com
CC: Szabolcs Nagy <nsz@...t70.net>
Subject: Re: ldso : dladdr support

Same as before except I use bit mask to support multiple hash algorithms.

On 08/08/2012 13:52, Szabolcs Nagy wrote:
> * musl <b.brezillon.musl@...il.com> [2012-08-08 11:55:45 +0200]:
>> Here is the new patch for dladdr + gnu hash support.
>>
>> On 08/08/2012 01:09, Rich Felker wrote:
>>> don't remember which) where I described some optimizations that should
>>> be present if gnu hash is to be supported. Basically, the dynamic
>>> linker should keep track of whether there's one of the two hashes
>>> that's supported by ALL loaded libs, and in that case, only ever use
>>> that hash, to avoid computing two different hashes.
>> The hash for given algo is only computed once (if needed).
>> That's the reason for the computed table.
>> If all libs uses the same hash algo, the other will never be computed.
> a lib can support both hash tables
> (so it's non-trivial to use the one that all libs support,
> eg instead of a single alg index a bit mask should be used)
>
See patch changes.
>> +#define SYSV_HASH_ALG_IDX   0
>> +#define GNU_HASH_ALG_IDX    1
>> +#define HASH_ALG_CNT		2
> if we go with this approach then use shorter names
> (thes are only used locally)
>
> eg. SYSV_HASH, GNU_HASH, NHASH
Corrected.
>> +	for (h1 &= (uint32_t)-2;; sym++) {
>> +		h2 = *hashval++;
>> +		if ((h1 == (h2 & ~1)) && !strcmp(s, strings + sym->st_name))
> these is still a ~1
>
> looking at it now probably writing out & 0xfffffffe
> is the cleanest
>
>> +	static uint32_t precomp[HASH_ALG_CNT][3] = {
>> +	  {0x6b366be, 0x6b3afd, 0x595a4cc},
>> +	  {0xf9040207, 0xf4dc4ae, 0x1f4039c9},
>> +	};
>> +	uint32_t h[HASH_ALG_CNT];
>> +	uint8_t computed[HASH_ALG_CNT] = {0, 0};
>> +	uint8_t alg = dso->hashalg;
>> +	if (alg == SYSV_HASH_ALG_IDX)
>> +		h[alg] = sysv_hash(s);
>> +	else
>> +		h[alg] = gnu_hash(s);
>> +
>> +	computed[alg] = 1;
>> +
>> +	if (h[alg] == precomp[alg][0] && !strcmp(s, "dlopen")) rtld_used = 1;
>> +	if (h[alg] == precomp[alg][1] && !strcmp(s, "dlsym")) rtld_used = 1;
>> +	if (h[alg] == precomp[alg][2] && !strcmp(s, "__stack_chk_fail")) ssp_used = 1;
>> +
>>  	for (; dso; dso=dso->next) {
>>  		Sym *sym;
>> +
>>  		if (!dso->global) continue;
>> -		sym = lookup(s, h, dso);
>> +
>> +		alg = dso->hashalg;
>> +		if (!computed[alg]) {
>> +			if (alg == SYSV_HASH_ALG_IDX) {
>> +				h[alg] = sysv_hash(s);
>> +				sym = sysv_lookup(s, h[alg], dso);
>> +			}
>> +			else {
>> +				h[alg] = gnu_hash(s);
>> +				sym = gnu_lookup(s, h[alg], dso);
>> +			}
>> +			computed[alg] = 1;
>> +		} else {
>> +			if (alg == SYSV_HASH_ALG_IDX)
>> +				sym = sysv_lookup(s, h[alg], dso);
>> +			else
>> +				sym = gnu_lookup(s, h[alg], dso);
>> +		}
> instead of arrays i'd write
>
> alg = dso->hashalg;
> if (alg == SYSV_HASH) {
> 	if (sysv_ok) {
> 		...
> 		sysv_ok = 1;
> 	}
> 	sym = sysv_lookup(s, sysv_h, dso);
> } else {
> }
Haven't changed it yet.
> since there are many ifs anyway
>
> the table approach is nicer when all data and functions
> are in a table:
>
> if (!ok[alg]) {
> 	h[alg] = hash[alg](s);
> 	ok[alg] = 1;
> }
> sym = lookup[alg](s, h[alg], dso);
>
>> +	p->hashalg = SYSV_HASH_ALG_IDX;
>>  	p->strings = (void *)(p->base + dyn[DT_STRTAB]);
>> +	for (; v[0]; v+=2) if (v[0] == DT_GNU_HASH) {
>> +		p->hashtab = (void *)(p->base + v[1]);
>> +		p->hashalg = GNU_HASH_ALG_IDX;
>> +	}
> so it seems gnu hash is used whenever it's present
> i'm not sure if that's the right default..
>
> another possibility is to have a plain find_sym function
> which is simple and only supports sysv hash
> and whenever it encounters a lib that has no sysv hash in
> it a find_sym_gnu is called that does the hard work
> (so using gnu only libs is penalized, i don't know how
> common that case is though)


View attachment "ldso-add-dladdr-and-gnu-hash-support.patch" of type "text/x-patch" (10516 bytes)

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