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Message-ID: <20120808115202.GL30810@port70.net>
Date: Wed, 8 Aug 2012 13:52:02 +0200
From: Szabolcs Nagy <nsz@...t70.net>
To: musl@...ts.openwall.com
Subject: Re: ldso : dladdr support
* 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)
> +#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
> + 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 {
}
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)
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